>/ 


v 


ON  THE 


GEOLOGY    OF    THE    DELTA, 

AND  THE 

MUDLUMPS  OF  THE  PASSES  OF  THE  MISSISSIPPI. 

BY  EUG.  W.  HILGAKD. 


[Read  before  the  American  Association  at  its  last  meeting  in  Troy.] 


[FROM  THE  AMERICAN  JOURNAL  OP  SCIENCE  AND  ARTS,  VOL  I,  1871.] 


In  previous  papers*  read  before  this  Association,  I  have  com- 
municated the  results  obtained  during  two  successive  geological 
expeditions  to  Louisiana,  so  far  as  the  more  ancient  formations 
are  concerned.     It  is  the  object  of  the  present  communication 
to  present  and  discuss  the  phenomena  of  that  portion  of  the 
*.  Jj^f pi tetyj  explored  by  me,  lying  within  what  is  usually  con- 
"•'si&feficL  .the  alluvial  area,  proper  or  Delta,  of  the  Mississippi 
.••pweii   .Mosrt  of  these  observations  were  made  in  1867,  during 
•"  *  the,  jji^t-.of. "these  excursions  referred  to,  under  the  auspices  of 
the   Smithsonian  Institution ;    their  publication   having   been 
delayed  in  consequence  of  want  of  time,  on  my  part  to  carry 
out  the  large  amount  of  chemical  and  microscopic  work  in- 
volved in  the  discussion,  which  is  even  yet  far  from  being  as 
full  as  would  be  desirable.     I  hope,  however,  to  be  enabled, 
hereafter,  to  continue  the  investigation  of  the  subject,  both  in 
the  field  and  in  the  laboratory. f 

I.    The  Upper  Delta.  Plain. 

I  recall  to  mind  the  fact  that,  as  we  descend  the  Mississippi, 
the  older  strata  successively  sink  from  view.  A  few  miles 
below  Vicksburg  we  lose  sight  of  the  older  Tertiary.  Thence 
down  to  the  latitude  of  Tunica  Bend,  La.,  we  find  the  rocks 
of  the  Grand  Gulf  (Tertiary)  age,  possessing  but  a  very  faint 
southward  dip.  Next,  the  oldest  representative  of  the  quater- 
nary epoch,  viz.,  the  stratified  Drift  or  Orange  Sand,  disappears 
beneath  the  water's  edge  near  Port  Hudson ;  while  the  swamp, 
lagoon  and  fluviatile  beds,  which  have  given  notoriety  to  the 
latter  locality,  are  seen  above  high  water  level  not  much  farther 
south  than  the  city  of  Baton  Rouge. 

The  gradual  descent  and  successive  disappearance  of  these 
strata  is  not,  however,  altogether  a  phenomenon  of  dip,  in  the 

*  This  Journal,  II,  vol.  xlvii,  Jan.,  1 869 ;  Ibid,  xlviii,  Nov.,  1 869. 

f  For  material  assistance  in  the  investigations  embraced  in  this  paper,  I  am 
especially  indebted  to  the  officers  of  the  Delta  Survey  in  charge  of  the  Coast  Sur- 
vey schooner  Varina — Messrs  F.  P.  Webber  and  Henry  L.  Marinden,  for  reports 
of  observations,  and  specimens  furnished;  to  Capt.  Day,  in  command  of  the  same 
vessel,  then  moored  at  the  Head  of  the  Passes,  for  a  boat's  crew  and  personal 
assistance  in  the  examination  of  the  mudlumps ;  to  Mr.  Moulton,  of  the  Cromwell 
line  of  steamers;  to  Captain  Ed.  Yorke,  of  the  Towboat  Association,  and  to 
Capt.  Andrews,  then  of  the  dredgeboat  at  the  Southwest  pass,  for  free  transporta- 
tion on  their  respective  vessels ;  and  to  Capt.  Tilford  and  other  gentlemen  of  the 
N.  0.  Pilot  Association,  for  generous  hospitality  as  well  as  much  valuable  informa- 
tion. For  similar  favors  I  am  indebted  to  Dr.  Copes,  President  of  the  N.  0.  Acad. 
of  Sciences,  as  well  as  other  members  of  that  body.  Other  acknowledgments 
will  be  found  in  their  proper  place. 


and  the  Mudlumps  of  the  Mississippi.  339 

usual  sense,  but  in  a  great  measure  the  result  of  consecutive 
deposition  ;  while  the  surface  slope  of  the  Port  Hudson  deposits 
is  manifestly  in  a  great  degree  due  to  denudation,  and  in  part, 
no  doubt,  to  deposition  on  a  sloping  bottom.  At  Port  Hudson 
as  well  as  elsewhere,  where  extensive  profiles  can  be  seen,  the 
deposits  of  that  era  exhibit  the  basin  shape ;  both  on  the  large 
and,  sometimes,  on  the  small  scale.  In  this  as  well  as  in  their 
lithological  and  paleontological  features,  they  greatly  resemble 
deposits  now  forming  over  large  areas  ;  and  where  the  two  are 
in  juxtaposition,  it  is  often  difficult,  sometimes  impossible,  to 
draw  the  line  between  them,  since  qualitatively  their  process  of 
formation  has  been  manifestly  the  same.  Both  above  and 
below  Port  Hudson,  and  down  as  far  as  Fort  St.  Philip,  the 
apparently  alluvial  river  banks  frequently  exhibit  at  low  water 
edge,  solid  blue  clays,  with  cypress  stumps  and  twigs  im- 
bedded therein,  scarcely  distinguishable  from  some  materials 
occurring  at  Port  Hudson,  Cote  Blanche,  and  other  localities 
of  the  Port  Hudson  age ;  and  as  neither  can  be  expected  to 
contain  any  but  living  organisms,  it  is,  thus  far,  from  general 
considerations  alone,  that  we  can  hope  to  deduce  their  real  age. 
It  might  seem,  at  first  sight,  that  the  distinction  is  practically 
of  little  moment;  but  when  it  is  considered  that  the  Port 
Hudson  deposits  are  separated  in  time  from  those  of  the  pres- 
ent era,  by  a  large  portion  of  the  "  Champlain  "  period  of 
depression,  plus  the  entire  "  Terrace  "  period  of  elevation,  it 
becomes  obvious  that  the  distinction  is  one  of  no  little  theoret- 
ical, and  some  practical,  interest.  For  while  the  Port  Hudson 
strata  yield  to  the  augur  almost  invariably  a  considerable  rise 
of  artesian  water,  no  such  result  can  usually  be  looked  for  in 
either  river  or  delta  deposits. 

I  think  that  a  retrospective  view  of  the  geological  history 
of  the  lower  Mississippi  Valley  and  Gulf  Coast,  as  developed 
by  my  observations  in  the  States  of  Mississippi  and  Louisiana, 
will  serve  to  show  the  probability  that  by  far  the  greater  portion 
of  what  now  constitutes  the  alluvial  plain  of  the  lower  Missis- 
sippi, is  covered  by  the  river  deposits  to  a  comparatively  insig- 
nificant depth  only  ;  excepting  where  the  ever  shifting  river 
channel  itself  has  caused  an  unusual  depth  by  excavation  and 
subsequent  filling  up. 

I  have  shown  that  toward  the  close  of  the  Drift  period,  the 
place  of  the  present  Mississippi  was  occupied  by  what,  but  for 
its  stupendous  proportions,  might  be  termed  a  torrent  of  fresh 
water,  having,  even  as  far  south  as  the  present  coast  line,  a 
velocity  sufficient  to  transport  pebbles  of  five  to  six  ounces 
weight,  from  localities  not  nearer  than  Tennessee  and  northern 
Arkansas;  together  with  the  smaller  ones  derived,  doubtless, 


970962 


340  E.  W.  Hilgard— Geology  of  the  Delta, 

from  the  same  sources  as  the  drift  boulders  of  Missouri  and 
Illinois.* 

In  the  bored  wells  of  Calcasieu,  these  pebbles  have  been 
found  as  much  as  450  feet  below  tide  level ;  the  inevitable 
inference  being  (provided  the  sea  level  remained  constant), 
that  since  the  time  of  their  transportation,  the  coast  has  suf- 
fered a  depression  to  at  least  that  extent — in  matter  of  fact, 
probably,  fully  twice  that  amount f  The  thickness  of  the 
drift  stratum  is  about  one  hundred  feet,  the  materials  growing 
finer  toward  the  top ;  indicating,  therefore,  a  diminished 
velocity  of  the  depositing  current. 

Overlying  this  sand  and  pebble  drift,  we  find,  as  at  Port 
Hudson,  alternating  strata  of  more  or  less  lignitiferous  clay 
and  sand,  850  feet  thick;  a  130-foot  clay  bed  lying  on  top.- 
In  its  uppermost  portion,  this  bed  recalls  to  mind  at  once  the 
variously  colored  clays  of  the  Cote  Blanche  profile,  with 
their  calcareous  concretions;  but  here,  according  to  the  con- 
current testimony  of  the  inhabitants  and  the  microscope,  marine 
shells  take  the  place  of  the  fresh-water  fauna  observed  at  the 
former  locality.  The  sands  which,  on  the  whole,  predominate 
in  the  lower  portion,  are  readily  distinguished  under  the  micro- 
scope from  those  of  the  drift,  by  the  predominant  sharpness  of 
the  grains,  and  the  occurrence  of  particles  of  lignitized  wood  ; 
but  I  have  not  thus  far  succeeded  in  finding  in  them  any  other 
organisms.  They  resemble  strikingly  the  materials  obtained  at 
corresponding  depths  in  the  bored  wells  in  the  city  of  New 
Orleans. 

Few  deep  wells  exist  near  the  coast,  in  the  interval  between 
that  city  and  the  Calcasieu  bores.  In  the  only  one  of  which  I 
possess  definite  data,  viz.,  at  Salt  Point  on  Bayou  Sale,  a  bed 
of  marine  shells  was  found  after  passing  through  the  first  clay 
bed,  at  60  feet. 

*  Miss.  Rep.,  1860,  p.  26  andff.;  this  Journal,  II,  May,  1866;  and  Nov.,  1866; 
Ibid  Jan.,  1869;  Ibid,  Nov.,  1869. 

The  magnitude  of  this  phenomenon  would  give  it  a  continental  significance 
even  if  it  were  confined  to  what  I  have  termed  the  "  Orange  Sand  delta,"  below 
Cairo.  But  as  I  have  elsewhere  stated,  similar  pebhle  streams,  with  slack  water 
deposits  of  ferruginous  sands  intervening,  exist  in  Texas ;  and  I  have  the  satis- 
faction of  adding  to  the  array  of  facts  heretofore  presented  in  connection  with 
this  subject,  the  testimony  of  Prof.  Safford,  of  Tennessee;  who,  upon  discussion, 
finds  that  the  phenomena  presented  by  the  "  Bluff  gravel,"  "Ore  region  gravel," 
and '-Eastern  gravel "  of  his  report  are  most  satisfactorily  accounted  for,  and 
brought  under  a  common  point  of  view,  upon  the  basis  set  forth  in  my  publica- 
tions, above  referred  to. 

In  connection  with  Tuomey's  and  my  own  observations  in  Alabama,  and  farther 
east,  those  of  Prof.  Safford  acquire  additional  significance,  and  the  southern  strati- 
fied drift  an  additional  cLiim  upon  the  serious  attention  of  American  geologists. 
Compare  Prof.  Newberry's  interesting  paper  on  "The  Surface  Geology  of  the 
basin  of  the  Great  Lakes,  and  the  Valley  of  the  Mississippi,"  where  it  is  stated 
that  "no  deposits  corresponding  to  the  drift  of  the  northern  and  western  states, 
exist  south  of  the  Ohio  river,"  (Ann.  Lye.  Nat.  Hist,  N.  York,  ix,  213,  1869). 

f  This  Journal,  II,  Nov  ,  1869,  p.  335. 


and  the  Mudlumps  of  the  Mississippi.  341 

The  same  is  true  of  the  formation  skirting  the  coast  of  Mis- 
sissippi Sound.  Deep  wells  there  sometimes,  though  not  al- 
ways, strike  beds  of  marine  shells,  and  water  possessing  a  con- 
siderable rise,  after  penetrating  the  uppermost  clay  bed  ;  which 
there  usually  also  contains  cypress  stumps,  and  forms  the  "  blue 
cla^y  bottom  "  of  the  Gulf  coast.  Sometimes,  though  rarely,  de- 
posits of  marine  shells,  of  living  species,  appear  in  the  beds  of 
streams.  But  so  far  as  I  know,  their  occurrence  is  limited  to 
within  a  moderate  distance  from  the  general  coast  line  ;  so  that 
the  great  body  of  the  formation  underlying  the  upland  parishes 
of  East  Louisiana,  the  Attakapas  and  Calcasieu  prairies,  as 
well  as  the  coast-belt  of  prairies  in  Texas,  consists  of  marsh, 
lagoon,  and  fluviatile  deposits,  with,  probably,  many  an  inlet 
or  estuary  of  a  more  or  less  brackish  or  marine  character. 

Obvious  as  is  the  conformation  of  this  littoral  belt  to  the 
outline  of  the  Gulf  coast,  the  Mississippi  valley  influences  it 
only  in  so  far,  as  that  its  strata  have  here,  probably,  their 
highest  absolute  elevation,*  and  farthest  extension  northward. 
The  latter  circumstance  is  the  natural  consequence  of  the  exist- 
ence of  the  depression  which,  at  least  since  the  opening  of  the 
Cretaceous  period,  has  determined  the  outline  of  the  formations 
southward  of  Cairo — an  embayment  which,  with  every  succeed- 
ing period  of  deposition,  became  less  concave,  until  at  the 
close  of  the  Grand  Gulf  epoch,  the  concavity  had  all  but  dis- 
appeared. During  the  period  of  slow  depression  which  charac- 
terized the  Port  Hudson  era,  the  present  general  coast  line  must 
have  been  established ;  and  when  upon  the  reversal  of  the 
movement  of  subsidence,  the  waters  of  the  continent  began  to 
be  discharged  through  what  is  now  the  lower  Mississippi  valley, 
the  erosion  seems  to  have  been  checked  everywhere,  save  per- 
haps in  the  main  channel,  by  the  tough  cypress  swamp  clay 
which  now  forms  the  immediate  substratum  of  the  lower  littoral 
belt,  and  extends  far  into  the  waters  of  the  Gulf. 

It  is  incredible  that  the  deposition  which  occurred  along  the 
whole  Gulf  coast  from  Indianola  to  Mobile,  should  not  have 
taken  place  also  in  the  main  axis  of  the  depression  which,  as 
the  trend  of  the  formations  shows,  had  in  a  great  measure  been 
filled  up.  Yet,  inasmuch  as  this  was  the  deepest  portion  of  the 
area,  it  is  to  be  expected  that  here,  if  anywhere,  marine  deposits 
should  extend  far  inland.  We  shall  not  therefore  be  surprised 
to  find  that,  as  Pourtales  has  proven  from  the  soundings  made 
under  the  direction  of  Gen.  Humphreys,  the  Mississippi  river 
flows  on  marine  beds,  at  New  Orleans  and  Bonnet  Carre.  It 
would  rather  be  remarkable  if  such  beds  should  not  appear 
even  much  higher  up  the  river,  since  even  in  the  comparatively 

*  As  regards  the  main  body.  At  Weeks'  Island  and  Petite  Anse,  and  proba- 
bly at  Cote  Blanche  and  Orange  Island,  their  elevation  exceeds  that  at  Port  Hud- 
son bluff. 


342  &   W.  Hilgard — Geology  of  the  Delta, 

insignificant  valley  of  Pearl  river,  they  have  been  met  with  in 
the  latitude  of  Baton  Rouge,*  about  30  miles  from  the  coast. 

It  is,  doubtless,  owing  to  the  formation  of  these  swamp 
deposits,  and  their  subsequent  resistance  to  denudation  during 
and  since  the  Terrace  epoch  of  elevation,  that  the  main  body  of 
the  truly  alluvial  delta  is  thrown  so  far  beyond  the  general  coast 
line,f  out  into  the  Gulf.  It  is  the  shallow  "blue  clay  bottom," 
so  well  known  to  navigators  on  the  Gulf  coast,  which  forces 
the  great  river  to  advance  its  mouths  so  rapidly  toward  deep 
water,  by  the  accumulation  of  its  own  deposits ;  and  the  bor- 
ings made  at  New  Orleans  have  shown  how  slight  is  the  thick- 
ness, even  at  such  an  advanced  point,  of  the  river  deposits 
proper,  overlying  the  older  formation. 

I  owe  to  the  active  interest  taken  in  this  subject  by  Gen.. 
A.  A.  Humphreys,  U.  S.  A.,  an  opportunity  of  examining, 
not  only  the  specimens  collected  during  the  boring  of  the 
artesian  well  at  New  Orleans  by  a  committee  of  the  New 
Orleans  Academy  of  Sciences  (so  far  as  they  were  preserved 
from  destruction  during  the  war) ;  but  also  those  obtained  in 
the  soundings  made  by  the  delta  survey  under  his  charge,  upon 
which  a  very  able  and  minute  report  of  a  microscopic  examina- 
tion had  previously  been  made  by  Mr.  L.  R  Pourtales.  Upon 
the  strength  of  the  data  furnished  by  the  latter,  as  well  as  by  the 
profile  constructed  by  the  committee  of  the  Academy  (repro- 
duced in  the  "  Report  on  the  Physics  and  Hydraulics  of  the 
Mississippi  river  "),  Gen.  Humphreys  concluded  that  at  Bonnet 
Carre  and  New  Orleans,  the  river  flows  on  an  ancient  sea  bot- 
tom, which  he  conjectured  to  be  of  Tertiary  age.  Sir  Charles 
Lyell  having  questioned  the  correctness  of  this  view,  Gen. 
Humphreys  obtained  from  the  N.  0.  Academy  as  complete  a 
suite  of  specimens  of  the  borings  as  could  be  collected,  and 
referred  them  to  me  for  examination.  The  first  results  of  this 
investigation  are  given,  in  substance,  in  the  first  volume  of 
Ly ell's  Principles  of  Geology,  10th  edition,  p.  459  ;  they  were 
based  substantially  upon  the  determination  of  the  visible  shells 
(mollusks)  contained  in  several  of  the  specimens,  embracing, 
fortunately,  most  of  the  important  horizons  mentioned  in  the 
profile.  I  have  since  gone  over  the  whole  ground,  in  the  micro- 
scopic examination  of  all  the  available  specimens,  with  a  view 
to  determining  their  (marine  or  fresh  water)  character,  and  the 
admissibility  of  the  supposition  that  they  might  belong  to  the 
delta  formation  proper. 

My  detailed  report  of  this  examination  will,  I  presume,  be 
published  before  Iong4  Unfortunately,  most  of  the  specimens 

*  Miss.  Rep.,  1860,  p.  156. 

f  Drawn,  say  from  the  mouth  of  Pearl  river  to  Belle  Isle,  the  most  advanced 
outpost  of  the  Port  Hudson  deposits  on  the  Louisiana  coast ;  which  line  will  pass 
near  the  city  of  New  Orleans. 

\  In  Rep.  of  the  U.  S.  Engineer  Dept.,  for  1870. 


and  the  Mudlumps  of  the  Mississippi.  343 

representing  the  important  clay  strata  (of  34,  32-J,  39,  and  63-J, 
feet  respectively),  were  missing  ;  but  as  regards  the  rest,  51  in 
number,  I  found  almost  all  derived  from  a  lower  level  than 
SI  feet,  either  characterized  by  marine  organisms  (shells,  corals 
or  foraminifera),  or  of  such  a  character  as,  by  their  obvious 
connection  with  the  others,  to  put  them  in  the  same  category, 
although  devoid  of  fossils.  I  cannot  omit  to  mention  in  this 
connection,  the  extraordinary  scarcity  of  marine  organisms  in 
some  of  the  specimens  brought  up  in  sounding  off,  on  and 
inside  the  bars  of  the  Missiasippi  Passes ;  the  most  patient 
search,  even  after  concentration  by  washing,  having  failed  to 
bring  to  light  anything  but  minute  fragments  of  wood,  root 
and  other  vegetable  fibers,  and  remnants  of  Naviculas.  These 
specimens  had,  it  is  true,  been  obtained  during  a  period  of  high 
water ;  and  others,  collected  at  corresponding  points  but  at  a 
different  season,  showed  abundance  of  foraminifera  and  even 
some  visible  shells.  But  in  view  of  these  facts,  it  is  quite  in- 
telligible how  in  an  estuarian  formation,  forming  at  the  outlet 
of  the  continental  waters,  many  portions  may  have  remained 
destitute  of  any  vestige  of  marine  life,  though  perhaps  deposited 
in  strongly  brackish  water ;  while  any  slackening  in  the  rate  of 
depression  would  promptly  cause  a  predominance  of  the  fresh 
over  the  salt  water,  a  stagnation,  and  consequent  deposition  of 
clays ;  which  would  be  nearly  or  quite  free  from  vestiges  of 
marine  life.  Such  is  the  case  in  some  of  the  few  clay  specimens 
from  this  bore,  which  I  have  had  the  opportunity  of  examin- 
ing; but  others  I  have  found  to  contain  not  only  foraminifera, 
but  abundance  of  shells.  In  the  clay  occurring  at  the  greatest 
depth  reached — 630  feet — Mr.  Pourtales  reports  an  abundance 
of  foraminifera. 

The  specimens  from  the  several  beds  contained  altogether 
about  50  species  of  mollusks,  of  which  40  were  in  such  a  con- 
dition as  to  be  determinable  with  certainty.  Of  these,  36  were 
species  now  living  in  the  Gulf,  and  4  were  new. 

The  latter  (belonging  to  the  genera  Gardium,  Abra,  Semele 
and  Tapes)  I  submitted  to  Mr.  Conrad  for  determination  and 
description.  He  remarks  that  while  they  (one  of  them  espe- 
cially) seem  to  be  rather  of  miocene  type,  and  not  known  to  be 
now  living  in  the  Gulf  of  Mexico :  yet  our  knowledge  of  the 
fauna  of  the  latter  is  so  imperfect  thus  far,  that  it  cannot  be  as- 
serted that  the  species  are  not  now  inhabitants  of  the  Gulf 
waters. 

As  regards  the  distribution  of  the  species,  there  is  no  mate- 
rial difference  from  the  highest  to  the  lowest  level,  the  leading 
and  predominant  species  being  everywhere  about  the  same,  and 
coinciding  in  a  marked  manner  with  the  fauna  collected  by 
myself  on  the  beach  of  Ship  Island  in  the  Mississippi  Sound ; 


344  K  W.  Hilgard— Geology  of  the  Delta, 

though  quite  different  in  the  prevalence  of  species,  from  that 
now  cast  ashore  on  the  islands  of  the  delta.  One  of  the  new 
species,  moreover,  occurs  abundantly  in  one  of  the  very  first 
shell-beds  ;  and  three  of  them  at  the  depth  of  235  feet,  as  well 
as,  in  part,  still  lower  down. 

In  view  of  all  the  facts  bearing  on  the  case,  the  most  probable 
conclusion  is  that  the  marine  formation  penetrated  in  the  New 
Orleans  well  is  altogether  independent  of  the  present  delta  for- 
mation ;  that,  on  the  contrary,  it  is  the  equivalent  in  time  of 
the  Port  Hudson  deposits,  which  everywhere  near  the  coast 
assume  a  marine  facies ;  and  would  necessarily  possess  that 
character  in  an  increased  degree,  where  the  deepest  depression 
existed. 

The  thickness  of  the  alluvium  proper  in  the  alluvial  plain 
will,  of  course,  vary  in  accordance  with  the  degree  of  denuda- 
tion that  the  older  formation  may  have  experienced  during  the 
era  of  upheaval ;  and  it  is  futile  to  attempt  an  estimate  of  the 
amount  of  alluvium  deposited  by  the  great  river  since  the 
beginning  of  the  modern  era,  until  numerous  observations 
shall  have  placed  us  in  possession  of  data  allowing  us  to  form 
an  approximate  estimate  of  its  depth  in  the  several  portions  of 
the  alluvial  plain.  While  there  exist  in  it,  doubtless,  a  num- 
ber of  ancient  river  channels,  we  already  have  proof  also  of 
the  existence  of  ridges  of  more  solid  and  ancient  ground,  far 
out  in  the  delta  plain,  which  seem  to  have  caused  the  eastward 
deflection  (parallel  to  the  Teche  and  the  main  Mississippi),  of 
Bayou  Lafourche  as  well  as  of  the  minor  channels.  I  have 
heretofore*  alluded  to  the  apparent  general  cause  of  this  de- 
flection, viz.,  the  barrier  of  drift  materials  accumulated, 
perhaps,  upon  a  Cretaceous  nucleus,  which  is  presented  by  the 
chain  of  Five  Islands— Belle  Isle,  Cote  Blanche,  Weeks'  Island, 
Petite  Anse  and  Orange  Island—  extending  from  Atchafalaya 
to  Yermilion  Bay. 

The  very  variable  depth  of  the  alluvium  is  well  exemplified 
by  the  borings  made  for  water  and  gas,  in  the  city  of  New 
Orleans,  by  Mr.  J.  B.  Knight,  of  that  city.  Its  lower  limit 
seems  to  be  almost  everywhere  marked  by  a  stratum  of  liquid 
mud,  beneath  which  appears  the  first  shell  bed.  It  is  from  this 
mud  stratum,  which  is  struck  at  depths  varying  from  31  (in 
the  well  of  1856)  to  56  feet,  that  combustible  gas  is  frequently 
found  to  issue  in  considerable  abundance,  and  with  a  pressure 
(as  reported  by  Mr.  Knight)  of  from  1-J  to  3  pounds  per  square 
inch.  The  discovery  (which  was  made  by  Mr.  Knight  in  bor- 
ing an  experimental  well  for  water,  on  his  premises)  at  first 
created  considerable  excitement,  as  it  was  thought  'the  natural 
gas  might  successfully  compete  with  that  of  the  gas  company, 

*  This  Journal,  II,  Jan.,  1869,  p.  88 ;  ibid,  Nov.,  1869,  p.  343. 


and  the  Mudlumps  of  the  Mississippi.  345 

which  it  was  said  to  equal  in  quality.  That  this  was  but  an 
indifferent  compliment  to  the  company's  product,  may  be 
judged  from  the  composition  of  the  natural  gas,  which  was 
analyzed  by  Prof.  J.  W.  Mallet,  then  of  the  University  of 
Louisiana,  with  the  following  result : 

Gas  from  "Knights1  well"  170  Gravier  street.  New  Orleans. 

Marsh  gas, 91  '81 

Carbonic  acid, _     2 '97 

Nitrogen, 5 -32 

Hydrocarbons  condensible  by  bromine, trace 

100-00 

The  gas  issued  at  the  rate  of  one  and  a  half  cubic  feet  per 
hour,  with  a  pressure  of  1  '6  inches  of  mercury.  It  was  reached 
at  a  depth  of  40  feet,  and  was  accompanied  by  a  considerable 
flow  of  faintly  saline  water. * 

Mr.  Knight  sunk  numerous  wells  in  different  portions  of  the 
city,  and  states  that  gas  was  struck  nearly  everywhere  at  depths 
varying  from  37  to  56  feet ;  its  amount,  in  one  and  the  same 
region,  being  sensibly  proportional  to  the  diameter  of  the  bore. 
In  a  few  cases,  an  extraordinary  amount  of  gas,  under  strong 
pressure,  was  struck.  In  an  article  published  in  the  New 
Orleans  Times,  of  March  19,  1870,  it  is  stated  that  "  at  the  old 
Washington  Artillery  building  on  Girod  street,  a  pipe  was 
driven  for  water,  and  the  gas  flowed  through  in  such  a 
volume,  that  when  ignited  it  fed  a  flame  15  feet  in  height, 
which  was  with'  difficulty  extinguished ;  and  when  it  was  at 
length  choked  out,  it  carried  up  several  cart  loads  of  sand  in  a 
single  night.  Yesterday  a  similar  phenomenon  presented  itself 
on  the  edge  of  the  sidewalk  in  Camp  street,  opposite  Lafayette 
square.  A  pipe  about  an  inch  and  a  half  in  diameter  had 
been  driven  into  the  ground  for  water,  when  at  the  depth  of 
60  feet,  a  rush  of  gas,  accompanied  by  water  and  sand,  was 
forced  through  to  a  distance  (height)  of  twelve  or  fifteen  feet 
above  the  top  of  the  pipe.  This  continued  for  two  hours,  in 
spite  of  all  efforts  to  suppress  it ;  and  the  result  was  a  deposit 
of  sand  mixed  with  fine  shells  and  pebbles,  amounting  to  at 
least  three  cart  loads.  Finally  the  workmen  succeeded  in 
closing  the  pipe,  and  forcing  it  through  the  gas-bearing 
stratum." 

The  supply  of  gas  was  in  the  end,  however,  thought  inade- 
quate for  practical  purposes  ;  while  the  original  object  of  obtain- 
ing drinkable  water  was  attained  to  a  limited  extent  only. 
Mr.  Knight  says  in  a  letter  on  the  subject,  that  he  has  "found 
great  irregularity  in  the  thickness  and  character  of  the  strata, 

*  See  analysis  beyond. 


346  E.  W.  Hilgard— Geology  of  the  Delta,  etc. 

and  it  is  impossible  to  tell  the  character  of  the  water  or  the 
depth  at  which  it  will  be  found,  before  trying.  At  two  places 
distant  about  1,500  feet,  I  obtained,  in  one,  at  the  depth  of 
48  feet,  a  free  supply  of  clear  water  strongly  impregnated  with 
iron  ;  in  the  other,  no  such  water  could  be  found  at  the  depth 
of  78  feet." 

Attention  having  been  called  to  the  subject,  a  "  find  "  of  gas 
was  next  reported  from  the  parish  of  Lafourche,  said  to  rise 
with  a  pressure  of  10  pounds  to  the  inch.  I  have  been  unable  to 
learn  whether  or  not  this  occurrence  of  gas  is  identical  with  that 
described  to  me  by  Col.  Thibodeaux,  of  Thibodeauxville,  as 
keeping  up  a  continual  agitation  of  the  waters  of  the  "Bayou 
bouillant,"  on  the  lower  Lafourche.  So  far,  no  practical  appli- 
cation of  this  source  of  gas  has  come  to  my  knowledge. 

As  regards,  then,  the  upper  delta  plain,  there  can  be  little 
doubt  that,  like  the  Calcasieu  and  Attakapas  prairies,  it  is 
underlaid  by  the  detrital  deposits  of  the  stratified  Drift,  at  a 
depth  which  may  fairly,  d  priori,  be  supposed  commensurate,  in 
a  measure,  with  the  importance  of  the  neighboring  channels ; 
viz.,  the  Sabine  on  one  hand,  and  the  Mississippi  on  the  other. 
About  midway  between,  the  Cretaceous  ridge  marked  by  the 
line  of  outliers  from  Lake  Bisteneau  to  Chicotville  or  Petite 
Anse,  has  caused  these  same  deposits  to  appear  at  the  surface.* 
The  overlying  swamp,  lagoon  and  estuarian  deposits  of  the 
Port  Hudson  age,  will  vary  both  in  thickness  and  in  the 
(marine  or  fresh-water)  character  of  their  materials.,  in  accord- 
ance with  the  conformation  (relative  to  the  ocean)  of  the  surface 
upon  which  they  were  deposited.  And  the  alluvial  deposits 
proper  will,  in  like  manner,  vary  in  thickness  in  accordance 
with  the  degree  of  denudation  previously  experienced  by  that 
older  formation,  but  appear  to  be  little  greater  on  the  alluvial 
plain  near  New  Orleans,  than  it  is  sometimes  found  to  be  in  the 
Yazoo  and  Tensas  bottoms,  f 

It  is  important  to  note  that,  under  this  point  of  view,  the 
ultimate  success  of  an  artesian  bore  at  New  Orleans  becomes  a 
matter  of  certainty — a  question  of  depth  alone.  All  water 
obtained  in  the  Port  Hudson  strata  possesses  considerable  rise, 
but  is  usually  too  strongly  mineral  to  be  desirable  for  every- 
day use.  The  waters  obtained  in  the  Orange  Sand,  on  the  con- 
trary, are  always  remarkably  pure,  and  when  struck  beneath  the 
Port  Hudson  deposits  cannot  fail  to  possess  a  proportionate 
rise,  as  in  Dr.  Kirkiman's  bore,  on  the  West  Fork  of  Calcasieu. 
The  waters  of  the  Port  Hudson  strata  would,  of  course,  require 
to  be  tubed  out. 

[To  be  continued.] 

*  This  Journal,  II,  Nov.,  1869,  pp.  332,  342  and  ff. 
f  Humphrey's  and  Abbot's  report,  pp.  98-100,  et  al. 


and  the  Mudlumps  of  the  Mississippi.  347 


II.    The  Lower  Delta  and  the  Mudlumps. 

A  glance  at  the  map  shows  that  in  descending  the  Mississippi 
from  New  Orleans,  we  find  a  narrow  strip  of  land  only  f  to  3 
miles  wide,  dividing  the  river  from  the  waters  of  the  Gulf;  from 
the  head  of  Oyster  Bay  opposite  Poirite  a  la  Hache  (about  half 
way  between  the  city  and  the  head  of  the  Passes),  down  to 
the  mouths.  Such,  at  least,  is  the  case  on  the  left  bank  ;  on  the 
right,  the  "neck"  begins  a  few  miles  below  Fort  Jackson. 
Down  to  the  forts,  the  aspect  of  the  "  Coast"  is  generally  pretty 
much  the  same,  where  its  original  character  has  not  been  lost  by 
cultivation  or  encroachment  of  the  river.  Nearest  the  river, 
and  highest  above  water  level,  are  the  sandy  "  willow  bat- 
tures,"  where  the  willow,  mingled  with  and  occasionally  re- 
placed by  the  cottonwood.  forms  the  predominant  growth.  Be- 
yond lies  a  belt  of  woodland,  timbered  chiefly  with  live-oak, 
magnolia,  and  cottonwood,  often  deeply  veiled  with  long- moss ; 
this  belt  embraces  the  richest  and  most  durable  soils  of  the 
"  Lower  Coast,"  and  is  mostly  occupied  by  magnificent  planta- 
tions of  sugar  cane  and  orange  orchards.  Beyond  these,  loom 
in  the  distance  the  sombre-hued,  moss-curtained  denizens  of 
the  cypress  swamp,  their  tops  forming  a  level  platform  sharply 
defined  against  the  horizon.  Between  the  swamp  and  the  wa- 
ter's edge,  seaward,  there  usually  intervenes  a  zone  of  reeds, 
with  here  and  there  a  stunted  cypress,  bay,  or  candleberry  bush, 
where  the  salt  water  has  but  slight  access. 

While  such  is  the  general  order  of  succession  of  these  belts 
of  vegetation  where  they  coexist,  either  or  both  of  the  two 
middle  ones  may  locally  be  absent  Such  is  alwaj-s  the  case 
where  the  uneck"  is  very  narrow,  as  happens  below  the  forts. 
Thence  to  the  mouths  of  the  passes,  the  willow  batture  and  the 
reed  marsh  alone,  with  few  exceptions,  form  the  barrier  be- 
tween the  river  and  the  sea  ;  it  is  traversed  by  numerous  small 
bayous,  some  of  which  are  in  great  part  the  work  of  the  duck- 
hunters  that  supply  the  New  Orleans  market,  and  whose  pur- 
suit leads  them  to  penetrate  the  marsh  for  the  purpose  of  reach- 
ing the  favorite  resorts  of  their  game.  These  bayous  increase 
in  frequency  as  we  descend,  and  in  approaching  the  mouths  of 
the  passes,  the  intervals  between  them  become  smaller,  until 
they  gradually  become  sheets  of  water  dividing  islands ;  and 
finally,  just  inside  the  bar,  we  have  the  latter  resolved  into  nu- 
merous individual  "mudlumps,"  dotting  the  surface  of  the  sea, 
on  both  sides  of  the  main  channel. 


348  E.   W.  Hilgardr— Geology  of  the  Delta,, 

Sir  Charles  Lyell  remarks  (Principles  of  Geology,  10th  ed., 
p.  448),  that  the  "phenomenon  of  the  mudlumps  is  without  par- 
allel, so  far  as  known,  in  the  delta  of  any  other  river.  The 
same  remark  might,  I  think,  appply  to  two  other  peculiarities, 
viz :  the  protrusion  of  the  long  neck  of  land  into  the  Gulf ; 
and  the  fact  that,  after  failing  to  send  out  any  branch  of  impor- 
tance for  a  hundred  miles  the  great  river  suddenly  divides  at 
one  point  into  three  widely  divergent  branches,  the  middle  one 
of  which  (the  South  Pass),  forming  the  direct  continuation  of 
the  channel,  is  the  smallest,  and  has  long  ceased  to  be  navigable. 
Evidently,  a  strong  extraneous  obstacle  alone  could  turn  aside 
the  powerful  current,  and  permanently  resist  its  erosive  and  un- 
dermining action.  And  now,  the  channel  which  carries  the 
main  current  (the  Southwest  Pass),  faithful  to  the  old  tradition, 
is  rapidly  pushing  out  into  the  Gulf  its  narrow  bands  of  reedy 
marsh,  without  a  branch  of  any  consequence  in  ten  miles  from 
the  head  of  the  Passes  to  the  light-house. 

A  glance  at  the  coast  lines,  as  well  as  at  the  intricate  ramifica- 
tions characterizing  the  deltas  of  the  Ehine,  the  Po,  the  Danube, 
the  Ganges,  or  the  Hoang-Ho ;  or  the  broad  inlets  forming  the 
mouths  of  the  rivers  of  South  America,  will  show  the  unique- 
ness of  the  Mississippi  mouths ;  the  Nile  and  the  Lena  alone 
exhibiting  a  general  form  at  all  analogous,  yet  very  distinct  in 
detail.  For  the  islands  off  the  Lena  mouths  are  not  "  mud- 
lumps  ;"  and  the  tongue  of  land  separating  Lake  Menzaleh  from 
the  Damietta  branch  of  the  Nile,  is  a  mere  sand-bar,  exhibiting 
no  analogy  save  that  of  form,  with  the  remarkable  u  necks"  of 
the  Mississippi  Passes. 

It  would  be  fair  to  infer,  a  priori,  that  some  connection  ex- 
ists between  the  exceptional  phenomenon  of  the  mudlumps, 
and  the  exceptional  form  of  the  delta ;  and  that  such  is  really 
the  case,  can  hardly  be  doubted  upon  a  candid  investigation  of 
the  facts.  So  far  from  being  an  unusual  phenomenon,  the  mud- 
lump-formation  appears  to  constitute  the  normal  mode  of  progression 
of  the1  Mississippi  mouths  ;  not  only  at  the  present  time,  but  for 
many  ages  past ;  perhaps  ever  since  the  broad  flood  of  the  Terrace 
epoch  subsided  into  the  present  Mississippi. 

The  characteristic  features  of  the  mudlumps  have  successively 
been  described  and  discussed  by  Sidell,*  Forshey,f  Chase. 
Beauregard  and  Latimer,$  Thornassy§  and  LyellJj  Yet  as  the 
phenomena  are  nowhere  described  in  their  entirety,  I  will  here, 
as  briefly  as  possible,  recapitulate  the  important  points. 

*  Report  to  Capt.  Talcott,  1839,  in  Humphreys  and  Abbott's  Report,  App.  A. 
f  MS.  Reporr.  1830. 

j  Keport  of  the  Board  of  Engineers  for  the  Examination  of  the  Mississippi;  Con- 
gress. Doc.  1852-53. 

§  Geologic  Pratiqu-de  IM  Louisiane,  1860,  Chap.  VI. 

I  Principles  of  Geology.  10th  edition,  1868;   vol.  1.  p.  449, 


and  the  Mudlumps  of  the  Mississippi. 


849 


350  E.   W.  Hilgard— Geology  of  the  Delta, 

The  mudlumps  originate  in  upheavals  of  the  bottom,  in  the 
region  lying  between  the  extreme  point  of  the  mainland  and 
the  crest  of  the  bar,  at  the  main  outlets  of  the  passes.  They 
all  lie  within  a  distance  of  from  one  to  three  miles  from  the 
axis  of  the  main  current,  and  nowhere  extend  into  the  bays  in- 
tervening between  the  several  active  mouths.  The  pilots  are 
under  the  impression  that  they  form  more  frequently  in  the 
main  channel  than  elsewhere  ;  but  allowance  must  be  made  for 
the  natural  infrequency  of  their  observations  outside  of  the  latter ; 
so  that  only  the  more  obvious  and  remarkable  changes  of  the 
bottom,  when  occurring  outside  of  their  regular  range,  would 
be  likely  to  come  under  their  notice.  A  priori,  it  would  seem 
probable  that  inasmuch  as  the  resistance  to  the  upheaving  force, 
other  things  being  equal,  must  be  less  in  deep  than  in  shallow 
water,  the  lumps  would  rise  more  frequently  and  more  rapidly 
in  the  channel  than  elsewhere.  On  the  other  hand,  the  denu- 
ding force  of  the  river  current  must  greatly  diminish  the  chan- 
ces of  any  such  upheaved  mass  appearing  above  the  surface,  or 
even  becoming  manifest  by  a  diminution  of  depth,  when  pro- 
gressing slowly.  Weighing  these  considerations  against  each 
other,  I  think  a  balance  remains  in  favor  of  the  pilot's  opinion  ; 
the  more  as  concurrent  testimony  goes  to  show  that  whenever 
the  channel  is  changed,  either  by  the  accumulation  of  deposit  or 
the  interference  of  a  mudlump  upheaval,  the  old  channel  is 
promptly  occupied  by  mudlumps  throughout  the  abandoned 
portion. 

As  regards  the  rapidity  of  the  upheaval,  it  appears  to  vary 
from  an  almost  secular  rate,  to  that  of  several  feet  in  24  hours. 
It  has  been  rumored  that  lumps  had  been  seen  to  rise  visibly, 
and  burst  open  like  a  bubble,  but  I  have  been  unable  to  trace 
the  statement  to  any  reliable  source,  and  it  is  discredited  by  the 
pilots.  The  most  rapid  rise  of  which  I  have  obtained  reliable 
information  was  witnessed  by  Capt.  Andrews,  of  New  Orleans. 
He  states  that  a  steamer  having  run  aground  about  nightfall  in- 
side the  bar,  her  bow  being  on  a  mudlump  in  about  2-J  feet  of 
water,  he  towed  her  off  during  the  night,  and  in  the  morning- 
found  the  imprint  of  her  bow  18  to  24  inches  above  water,  in  the 
soft  mud.  Allowing  for  the  possible  maximum  influence  of 
tides  in  favor  of  the  difference  observed,  the  minimum  rate  of 
upheaval,  according  to  these  data,  still  amounts  to  one  inch  per 
hour.  Nor  is  this  the  only  case  in  which  a  rapid  shallowing  of 
the  water  has  been  observed  as  a  consequence  of  the  grounding 
of  large  vessels.  This  has  usually  been  ascribed  to  the  acci? 
mulation  of  deposit  in  the  slack  water  so  formed,  and  in  many 
instances  this  may  have  been  the  true  cause.  But  this  explana- 
tion can  hardly  apply  to  the  case  detailed  above,  and  it  is  rea- 
sonable to  suppose  that  the  shock  of  a  grounding  vessel  may 


and  the  Mudlumps  of  the  Mississippi  851 

in  many  cases  prove  "  the  hair  that  breaks  the  camel's  back," 
when  acting  upon  a  portion  of  bottom  just  about  to  yield  to 
the  upheaving  force,  and  therefore  in  a  state  of  unstable  equi- 
librium. Future  observations,  however,  will  readily  settle  the 
question. 

Of  authentic  observations  illustrating  ordinary  rates  of  rising, 
I  quote  the  following  remarks  of  Mr.  H.  L.  Marindin,  U.  S. 
Coast  Survey,  who  in  1867,  while  engaged  in  the  survey  of  the 
mouths,  at  my  request  paid  considerable  attention  to  the  phe- 
nomena of  the  mudlumps,  and  made  an  interesting  report  to  me 
on  the  subject,  dated  June  20th,  and  accompanied  by  speci- 
mens. 

"  Since  the  first  examination  of  the  mudlumps  on  Southwest 
Pass  Bar,  there  have  appeared  numerous  lumps  in  the  vicinity 
of  the  main  channel,  whose  appearance  cannot  be  attributed  to 
other  causes  than  the  upheaval  of  the  bottom  by  some  unknown 
agency ;  since  tidal  observations  made  during  the  month  of  April, 
May  and  part  of  June,  show  no  material  decrease  in  the  height  of 
water  in  the  river,  other  than  the  daily  ebb  of  the  tide.  In  one 
instance,  also,  on  this  bar,  it  has  been  ascertained  from  surveys 
made  at  different  times,  that  where  in  the  month  of  April  a 
channel  with  fifteen  feet  of  water  existed,  there  is  now  a  lump, 
doubtless  still  forming,  with  only  six  feet  of  water  on  it  at  the 
highest  stage." 

Making  allowance  for  a  possible  accumulation  of  deposit,  the 
rate  of  rise  cannot,  in  this  instance,  be  estimated  at  less  than  two 
feet  per  month  ;  which,  so  long  as  the  lump  remains  under  wa- 
ter, seems  to  be  no  unusual  amount.  When,  however,  the  sur- 
face of  the  water  is  once  reached,  the  rising  becomes  slower  or 
ceases  altogether,  owing  probably  to  the  greater  weight  acquired 
by  the  material  upon  its  emergence.  At  least,  I  have  failed  to 
find  much  above  tide  level,  any  material  which  did  not  bear 
distinct  evidence  of  its  having  been  formed,  not  by  river  allu- 
vion, but  by  the  action  of  the  regular  mudsprings  ;  which,  if  not 
now  active  upon  all  the  lumps,  have  demonstrably  been  instru- 
mental in  forming  the  great  majority  of  the  masses  now  above 
water  level. 

I  do  not  know  that  any  one  has  ever  witnessed  the  first  out- 
burst of  a  mudspring  on  a  newly  risen  lump  ;  but  we  find  them 
in  all  stages  of  progress,  from  the  islet  bearing  its  first  tuft  of 
rushes,  to  the  active  cones  glistening  in  the  sunshine,  and  from 
time  to  time,  when  an  unusually  large  gas  bubble  rises,  spatter- 
ing the  liquid  mud  (which  usually  flows  in  a  quiet  stream)  all 
over  the  slopes ;  then  the  half-extinct  cone,  in  whose  crater  a 
little  pool  of  salt  water  is  at  long  intervals  agitated  by  a  gas 
bubble ;  then  the  extinct  and  collapsed  cone,  surrounded  by  a 
circular  moat  and  Somma-like  ring-wall ;  next,  the  old  lump  of 


352  E.  W.  Hilyard— Geology  of  the  Delta, 

jagged  outlines,  whose  disintegrated  materials  are  rapidly  yield- 
ing to  the  combined  attack  of  rain,  sunshine  and  waves,  till 
something  looking  like  a  large  stump  of  a  tree  is  all  that  re- 
mains of  an  island  of  several  acres ;  and  finally  the  shoal,  marked 
by  dangerous  rollers,  whose  surface,  on  a  calm  da}',  still  exhibits 
the  concentric  markings  corresponding  to  the  several  cones 
which  originally  built  up  the  island. 

I  regret  being  unable  at  the  present  time,  to  present  accurate 
sketches  of  these  several  stages  of  "  the  mudlumps'  progress."  I 
hope  to  do  so  hereafter,  but  for  the  present  must  confine  myself  to 
the  approximate  outline  representations  given  in  the  plate,  and 
a  brief  description  of  the  several  stages  as  observed  by  myself  in 
1857. 

Nascent  Lumps. — As  regards  the  first,  it  is  probable  that  a 
good  many  lumps  never  pass  beyond  that  stage  of  develop- 
ment, tor  the  reason  that,  so  soon  as  the  resistance  is  materially 
increased  by  the  emergence  of  a  portion  above  the  surface  of 
the  water,  the  upheaving  force  seeks  a  vent  elsewhere. 

Either  the  mud  or  gas-spring  breaks  out  beneath  the  water, 
and  becomes  perceptible  only  by  the  more  or  less  regular  and 
localized  evolution  of  bubbles  on  the  outskirts  of  the  lumps  ; 
a  very  common  phenomenon  in  the  neighborhood,  not  only  of 
new  and  active  lumps,  but  also  about  extinct  ones,  which  are 
in  course  of  demolition  by  the  waves.  On  the  extensive  sandy 
shoal  off  Stake  Island,  on  the  Southwest  Pass,  such  subaqueous 
gas-springs  may  be  observed  in  great  numbers. — Or  it  may 
happen,  that  another  portion  of  the  bottom,  now  offering  less 
resistance  than  the  mudlump,  will,  in  its  turn,  give  way  before 
the  upheaving  force,  till  the  same  degree  of  emersion  is  ob- 
tained, or  a  vent  is  opened. 

I  have  been  unable  to  ascertain  how  high  any  mudlumps  can 
rise  bodily  above  the  water  without  the  appearance  of  a  vent 
As  much  as  three  feet  has  been  observed  with  certainty ;  but 
unless  the  fact  that  it  is  a  new  upheaval  be  historically  known, 
it  must  be  extremely  difficult  to  ascertain  it,  unless,  by  actual 
access  to  the  interior,  it  can  be  shown  that  its  surface  strata  are 
old  river  deposits,  which  can  readily  be  distinguished  from 
those  formed  by  mudlump  vents.  Yet  these  might,  during  the 
elevation  of  the  lump,  have  been  removed  bv  the  current  At 
all  events.  I  have  failed  to  find  on  the  surface  of  any  lump 
much  above  tide- water,  anything  like  true  river  alluvium ;  the 
visible  material  being  either  such  as  is  now  formed  by  active 
vents,  or,  (as  on  the  lower  slopes),  that  which  obviously  results 
from  the  disintegration  of  the  former,  being  altogether  devoid 
of  structure. 

It  is  said  that  lumps  sometimes  sink  from  view  again  after 
bare  emergence.  I  know  of  no  authentic  example,  but  it 


and  the  Mudlumps  of  the  Mississippi.  353 

seems  likely  enough  that  upon  the  formation  of  a  large  vent  else- 
where, such  a  thing  might  happen ;  especially  if,  as  I  think 
probable,  mudsprings  and  mud-cones  form  beneath,  as  well  as 
above  the  water. 

Active  Cones. — In  the  second  stage,  that  characterized  by  the 
formation  of  the  active  eruptive  cones,  mudlumps  offer  an  as- 
pect so  strikingly  like  that  of  the  mud  volcanoes  of  Tuscany, 
as  to  stagger  the  observer's  geographical  consciousness.  The 
most  striking  example  of  the  kind  existed,  at  the  time  of  my 
visit,  off  Pass  a  1 'Outre,  on  the  south  (right)  side  of  the  channel. 
The  island*  is  about  an  acre  in  extent,  and  besides  the  active 
vents  (of  which  there  are  seven),  we  perceive  on  the  southern 
portion  the  remnants  of  long  extinct  craters,  in  various  degrees 
of  advancement  toward  old  age  and  degradation,  and  partially 
covered  with  vegetation. 

The  cones  on  the  north  or  channel  side,  at  a  distance,  present 
a  glistening  surface,  and  those  possessing  a  central  vent  only 
are  very  regularly  and  smoothly  conical.  Of  these  there  are 
four,  the  diameters  of  the  bases  ranging  from  eighteen  to  fifty 
feet,  and  their  elevation  from  one  to  two  and  a  half  feet  above 
the  general  level  of  the  island ;  making  the  angle  of  the  slope 
from  5°  to  8°  only,  instead  of  25°  to  35°,  as  given  in  the 
published  sketches.  I  have  seen  no  undisturbed  cone  whose 
slope  exceeded  about  12°.  The  slope  is,  of  course, '  essentially 
a  function  of  the  thickness  and  character  of  the  mud ;  which  in 
the  present  instance  flows  from  the  circular  basin  at  the  summit, 
4  to  8  ins.  in  diameter,  in  a  regular,  creamy  stream,  interrupted 
occasionally  only  by  a  gas  bubble  ;  which,  if  the  mud  be  thick, 
spatters  it  about  to  some  extent.  The  mud-stream  varies  from 
3  to  4  gallons  per  minute  in  the  largest  (eastern)  cone,  to  a  pint 
or  two  in  the  less  active  ones ;  sometimes,  in  running  down  the 
sides,  it  spreads  over  as  much  as  -J-  of  the  circumference,  but 
usually  forms  a  stream  4  to  8  inches  wide,  serpenting  down  the 
slope-  between  banks  formed  by  its  own  solidification,  as  does  a 
lava  stream.  As  these  banks  grow  in  height  by  drying,  on  the 
edge  of  the  crater,  they  gradually  compel  the  mud  to  rise 
higher  before  it  can  flow  off;  whereupon,  after  a  while  the 
column  overflows  at  another  point  of  the  circumference,  where 
the  same  play  is  then  repeated.  In  the  meantime,  the  previous 
mudstream  has  an  opportunity  of  consolidating,  drying  more 
or  less,  and  undergoing  a  variety  of  chemical  changes  depen- 
dent upon  the  character  of  the  water  and  the  duration  of  the 
exposure  to  the  air,  which  manifest  themselves  in  changes  of 
color  and  consistency ;  by  these  the  individual  streams  are 
distinctly  defined  from  one  another.  Each  one  forms,  of  course, 

*  It  was  named  Marindin's  Lump  by  the  crew  of  the  U.  S.  Coast  Survey  Schr. 
Varina,  and  I  shall  allude  to  it  under  that  designation. 


354  E.    W.  Hilgard—  Geology  of  the  Delta, 

a  more  or  less  irregular  portion  of  a  conic  surface,  the 
section  being  more  or  less  lenticular. 

There  results,  as  may  be  supposed,  a  very  peculiar  structure  or 
stratification,  unlike  anything  usually  seen  outside  of  volcanic 
districts,  unless,  perhaps  in  the  somewhat  analogous  case  of  the 
lee  side  of  dunes.  It  may  roughly  be  compared  to  the  upper 
half  of  an  onion.  It  is  needless  to  say  that,  once  seen,  it  cannot 
readily  be  mistaken  for  anything  else ;  and  its  absence  must  be 
held  as  proof  conclusive  of  an  absence  of  genetic  analogy.* 
The  thickness  of  the  layers  rarely  exceeds  1^  inches  ;  their 
colors  vary  from  dark  mouse-color  and  bluish  gray,  through 
dun  and  buff,  to  rust-color  and  red ;  the  materials,  from  hard, 
plastic  clay,  always  containing  a  great  deal  of  fine  silex  and 
more  or  less  coarse  sand,  to  sandy  clay  and,  rarely,  clayey 
sand ;  the  selvedges  are  frequently  marked  by  iron  rust  and 
mica  scales.  Where,  as  is  frequently  the  case,  neighboring 
cones  are  confluent  at  their  bases,  the  structure  is  of  course 
complicated  in  a  manner  readily  imagined. 

As  regards  the  gas  evolved  (which  is  in  all  cases  inflammable), 
the  small  proportion  its  bulk  bears  to  that  of  the  mud  simul- 
taneously ejected  (about  ^  to  ^  at  Marindin's  Lump),  at  once 
does  away  with  the  impression  mostly  entertained,  that  the  gas 
bubbles  bring  up  the  mud  with  them.  The  latter  comes  up 
with  a  steady  flow,  evidently  the  result  of  static  pressure,  and 
is  only  from  time  to  time  agitated  by  a  gas  bubble,  larger  or 
smaller  according  as  the  mud  is  more  or  less  consistent.  We 
sometimes  find,  about  extinct  cones  especially,  lively  gas 
springs  in  which  the  proportion  of  gas  is  considerably  greater, 
than  happens  in  any  active  cone  that  has  come  under  my 
observation  ;  but  in  that  case,  it  is  almost  always  accompanied 
only  by  water,  as  though  its  source  Were  above  the  stratum 
which  furnishes  the  mud.  For,  a  glance  at  the  river  deposits 
forming  around  the  lumps,, at  once  shows  that  they  are  totally 
different  from  the  fine,  clayey  material  of  which  the  cones  are 
formed  ;  nor  is  it  credible  that  the  mere  passage  of  a  current 
of  water  and  gas  through  such  deposits,  or  in  fact,  any  consolidated 
materials,  should  produce  such  a  perfect,  creamy  mixture  as 
that  ejected  from  these  craters. 

Extinct  Cones. — (rood  examples  of  cones  whose  activity  has 
nearly  or  quite  ceased,  may  be  seen  off  the  Northeast  Pass. 
Here  the  material  is  somewhat  sandier  and  firmer  than  the 
mud  ejected  at  Passe  a  1'Outre.  While  at  the  latter  place  it 
was  difficult  to  reach  the  craters  without  being  hopelessly 

*  Vide  Thomassy's  remarks  on  the  Five  Islands,  etc.,  Geol.  prat,  de  la  Louisiane, 
chap.  viii.  With  even  less  show  of  reason  a  similar  origin  has  repeatedly  been 
claimed  for  the  low  circular  mounds  which  dot  a  large  part  of  western  Louisiana. 
They  consist  exclusively  of  unstratified  sandy  materials,  and  are  doubtless  the 
result  of  animal  activity — probably  of  the  large  ant. 


and  the  Mudlumps  of  the  Mississippi.  355 

bogged,  they  can  readily  be  reached  almost  dry  shod  at  the 
Northeast  Pass.  Here,  also,  the  top  layers  were  peeling  off 
in  large  concave  "  flakes,"  from  the  effect  of  sun-cracks  and 
drying ;  such  a  surface  again  overflowed  by  mud  would  add 
another  singular  feature  to  the  structure  of  the  cones,  which 
may  also  be  noticed  in  many  sections  of  ancient  ones. 

The  vent  of  a  large  cone  in  this  region  (see  Plate)  formed  a 
basin  about  8  inches  diameter,  containing  a  puddle  or  salt  water 
covered  with  an  iridescent  ferruginous  pellicle,  disturbed  at  long 
intervals  by  small  gas  bubbles.  Evidently,  the  resistance  in  the 
large  cone,  elevated  about  10  feet  above  the  water  level,  had 
become  too  great  for  the  ejection  of  mud  ;  in  consequence 
whereof,  a  lively  little  cone  was  glistening  and  babbling  at  a 
level  several  feet  lower,  near  the  foot  of  the  old  cone.  But  the 
testimony  of  the  pilots  goes  distinctly  to  show,  that  the  active  cones 
become  more  lively,  and  dormant  cones  resume  their  activity,  at  high 
stages  of  water  in  the  river.  At  the  time  of  mv  visit,  the  water 
was  at  an  unusually  low  stage. 

Collapsed  Cones. — The  cones,  especially  the  larger  ones,  appear 
to  be  unable  to  survive  for  any  great  length  of  time  the  cessa- 
tion of  activity.  The  change  that  occurs  seems  to  be,  in  all 
cases,  a  sinking  of  the  central  portion,  often  to  such  an  extent 
that  its  place  becomes  occupied  by  a  pool  or  lagoon,  surrounded 
by  a  circular  rim  whose  strata  incline  away  from  the  center  at 
angles  invariably  much  steeper  than  is  found  in  any  recent 
cones,  from  20°  to  as  much  as  45°.  Outside  of  this^first  rim 
there  mostly  appears  a  series  of  concentric  crevasses,  sometimes 
several  feet  in  width  and  in  depth ;  and  the  annular  segments 
thus  brought  to  a  level  by  a  subsidence,  also  exhibit  the 
singular  feature  of  a  steeper  inclination  of  the  lines  of  deposi- 
tion, than  is  found  in  any  cones  now  forming.  The  seeming 
anomaly  of  this  fact  caused  me  to  observe  the  phenomena 
closely  ;  but  I  have  been  unable  to  find  an  exception  to  the 
rule ;  and  I  have  been  led  to  doubt  whether  i  n  some  cases, 
instead  of  true  eruption  cones,  these  collapsed  areas  and  their 
surroundings  may  not  be  the  remnants  of  the  original  up- 
heaved "bubble.'1  The  objection  to  this  interpretation  is  the 
character  of  the  material,  which,  excepting  in  a  few  cases  on 
the  Southwest  Pass,  is  altogether  unlike  the  present  river 
deposits,  both  in  structure  and  composition. 

A  most  perfect  exemplification  of  a  large  central  lagoon 
(100  feet  by  75)  surrounded  by  an  elevated  rim  and  several 
successive,  concentric  "moats,"  also  in  part  filled  with  water, 
occurs  on  one  of  the  numerous  mudlumps  S.W.  of  Stake 
Island,  on  the  Southwest  Pass  (see  Plate).  The  central  portion 
does  not  always,  however,  sink  out  of  sight ;  sometimes  a  conical 
mound  is  still  observable,  as  the  center  of  the  more  or  less  circular, 

c2 


356  K.   W.  Hilgard-  Geology  of  the  Delta, 

concentric  crevasses,  which  a  close  inspection  shows  to  exist  on 
almost  all  lumps  not  too  far  advanced  towards  decrepitude. 
Many  islands  exhibit  several  such  centers  and  systems  of 
crevasses,  indicating,  probably,  the  previous  existence  of  cones, 
sometimes  several  hundred  feet  in  diameter,  greatly  exceeding 
the  largest  now  in  a  state  of  activity,  both  in  diameter  and 
original  height. 

Degradation  and  Disappearance  of  Lumps. — The  direct  action 
of  the  waves,  unassisted  by  atmospheric  agencies,  produces  but 
little  effect  upon  the  yielding,  but  tough  and  coherent  material 
of  the  cones.  The  heaviest  breakers  and  rollers  spend  their 
force  in  vain  against  the  clay  shoals  which  render  most  of  the 
lumps  so  difficult  of  access.  The  unwary  will  often  be  tempt- 
ed by  the  deceptive,  rojk-like  aspect  of  this  material,  to  jump 
into  the  shallow  water  and  wade  ashore ;  but  a  plunge  knee- 
deep  into  the  apparent  solid  will  frequently  reward  his  temerity. 
Yet  so  long  as  this  mass,  which  shows  plainly  the  peculiar, 
concentrically  banded  mudlump  structure,  remains  constantly 
covered  with  water,  the  waves  rolling  over  it  produce  but  little 
impression. 

Not  so  with  the  portion  that  projects  above  water,  which  is 
alternately  exposed  to  rain,  sunshine,  and  the  wash  of  waves. 
Especially  where  the  material  is  clayey,  these  agencies  com- 
bined soon  produce  a  change  in  which  the  structural  as  well  as 
the  paleontological  characters  of  the  original  material  are 
totally  obliterated.  A  rain  falling  upon  a  fresh  surface  of  the 
latter,  causes  it  to  swell ;  then,  upon  exposure  to  sunshine,  it 
will  contract  into  prismatic  cleavage-forms.  A  slight  rain,  or 
the  spray  itself,  will  then  cause  the  extreme  surface  to  crumble 
into,  and  partially  fill  up,  the  cracks ;  when  the  swelling 
consequent  upon  a  thorough  wetting,  by  either  rain  or  waves, 
will  force  them  to  open  still  more,  while  streamlets  of  fluid 
rnud  follow  each  retiring  wave;  which,  perhaps,  has  thrown 
up,  and  left  high  and  dry  in  the  cracks,  a  dozen  species  of 
shells,  entirely  foreign  to  the  mucllump  mud  itself.  It  is  thus 
that  the  structureless,  tough  soil  of  the  general  surface,  and  of 
the  beach  of  the  mudlumps,  is  formed  ;  and  to  it  alone  apply 
the  descriptions  given  of  the  mass  of  the  mudlumps,  by  Sidell, 
Thomassy  and  Lyell,  as  a  "homogeneous,  tenacious  mud." 

A  frequent  repetition  of  this  process  involves,  of  course,  not 
only  a  rapid  direct  degradation  of  the  lump,  but  it  causes  it  to 
be  cleft  into  fragments  by  rents  gradually  progressing  from 
above  downward,  which  rapidly  increase  the  surface  exposed 
to  attack ;  and  eventually,  some  large  wave,  in  retiring,  carries 
down  with  it  a  huge  prismatic  slice,  leaving  behind  an  almost 
vertical  cliff.  This  is  carried  away  in  its  turn,  and  thus, 
attacked  from  all  sides,  an  island  of  considerable  size,  after 


and  the  Mudlumps  of  the  Mississippi.  357 

passing  through  a  variety  of  middle  stages  in  which  it  strik- 
ingly resembles  masses  of  trap  or  basalt,  magnified  into  distant 
mountains  by  the  peculiar  optical  delusion  prevailing  in  the 
region,*  is  finally  reduced  to  what,  at  a  distance,  appears  to  be 
the  stump  of  a  tree.  At  last,  some  storm  sweeps  away  this  last 
monument  of  the  disappearing  lump,  and  white-capped  rollers 
alone  mark,  thereafter,  the  higher  points  of  the  mud-shoal. 

I  have  best  observed  these  phenomena  of  disintegration  in 
every  stage  of  progress,  among  the  mudlumps  off  the  Northeast 
Pass,  where  the  fanciful,  cliff-like  forms,  figured  by  Thomassy, 
Sidell,  and  Lyell,  may  be  seen  to  perfection. 

It  is  said  that  there  are  no  mudlumps  off  South  Pass  or 
Grand  Bayou  ;  a  statement  which  may  require  to  be  taken  with 
a  grain  of  allowance,  but  agrees  with  the  general  impression 
that  the  mouths  discharging  the  largest  amount  of  water,  also 
exhibit  mudlump  activity  on  the  most  extensive  scale. 

The  Southwest  Pass  is  the  main  outlet  at  the  present  time  ; 
the  area  inside  the  bar  is  thickly  studded  with  mudlumps, 
chiefly  west  of  the  channel ;  and  as  before  stated,  lumps  have 
risen  there  repeatedly  under  the  eyes  of  the  pilots  and  survey 
parties.  Yet  there  is  not  there,  at  the  present  time,  a  single 
active  cone,  so  far  as  I  am  aware  ;  although  salt  water  and  gas 
springs  are  of  frequent  occurrence,  both  on  and  around  the 
islands.  There  is  a  marked  difference  between  the  river 
deposits  as  well  as  the  mudlump  materials  of  Passe  a  1'Outre 
and  Southwest  Pass,  the  latter  being  decidedly  more  sandy, 
and  sand  bars  taking  the  place  of  the  mud  flats  off  the  former. 
Whether  this  circumstance  (the  natural  result  of  the  greater 
swiftness  of  the  current  in  Southwest  Pass),  is  connected  with 
the  absence  of  active  cones,  it  may  be  too  early  to  discuss. 
Some  very  lively  springs  on  a  large  mudlump  off  Stake  Island 
on  that  Pass,  in  which  the  gas  emitted  is  about  equal  in  bulk 
to  the  water,  rise  in  small  basins  excavated  at  the  foot  of  a 
large  cone  which  must  have  been  15  to  18  ft.  high  ;  but  the 
material  they  bring  up  is  so  very  sandy  that  the  water  runs  off 
perfectly  clear.f 

Mudlumps  in  the  Marshes. — I  have  before  remarked,  that  at 
the  present  time,  the  upheaval  of  mudlumps  on  the  passes,  and 
subsequent  silting  up  of  the  shallows  between  them  (by  river 
deposit,  as  well  as  by  the  degradation  of  the  lumps  themselves,) 
seems  to  be  the  normal  mode  of  progression  of  the  delta.  The 
more  advanced  portions  of  the  narrow  bands  of  shore  now 

*  Caused  probably  by  a  faint  bluish  haze,  through  which  an  island  a  quarter  of 
a  mile  distant  and  15  feet  high,  appears  to  be  a  wooded  mountain  with  rocky 
escarpments,  and  at  its  foot  a  wide  spreading  city — which  suddenly  resolves  itself 
into  a  row  of  grave  white  pelicans  perched  on  the  beach  and  taking  wing  at  the 
approach  of  a  boat. 

f  See  analysis  of  the  water  of  these  springs,  below ;  marked  u  S.  W.  Pass,  I." 


358  E.  W.  Hilgard— Geology  of  the  Delta, 

forming  along  each  one  of  the  passes,  are  historically  known  to 
consist  of  mudlump-chains ;  and  in  the  absence  of  any  plausi- 
ble presumption  to  the  contrary,  as  well  as  of  any  parallel 
example  in  other  rivers,  it  is  reasonable  to  surmise,  that  not 
only  the  shores  of  the  present  passes,  but  also  the  neck,  at  least 
from  Pointe  a  la  Hache  down,  owes  it  formation  and  peculiar 
features  to  the  same  agencies. 

We  have  seen  how  rapidly  and  completely  the  joint  action  of 
the  waves  and  atmospheric  agencies  accomplish  the  degradation 
of  elevated  lumps  to  the  common  level  of  the  tide ;  and  where 
the  nature  of  the  materials  is  such  as  to  yield  readily  to  these 
destructive  influences,  it  would  be  unreasonable  to  look  for 
vestiges  of  ancient  lumps  above  that  level.  Such  is  the  case, 
as  before  mentioned,  on  Northeast  Pass  and  its  branches.* 
But  the  sandier  nature  of  the  mudlump  mass  on  Southwest 
Pass  enables  it  to  resist  much  longer,  so  that  some  of  the  larger 
and  more  elevated  islands  there  seem  destined  to  retain,  more 
or  less  permanently,  their  present  form.  For  the  same  reason, 
perhaps,  the  Southwest  Pass  furnishes  the  one  prominent  ex- 
ample of  the  existence  of  an  active  and  characteristic  mudlump, 
in  the  level  marsh  on  the  right  of  the  channel,  about  five  miles 
below  the  Head  of  the  Passes,  and  seven  above  the  mouth 
(light-house) ;  distant  1-J-  miles  from  the  river  bank,  and  about 
one  mile  from  the  beach  of  West  Bay.  Double-headed  Bayou, 
or  one  of  its  channels,  passes  within  a  few  hundred  yards 
of  this  lump ;  which  is  so  difficult  of  access  that  it  has  been 
very  rarely  visited,  though  plainly  visible  from  the  hurricane 
deck  of  passing  steamers,  from  which  I  have  examined  it  with 
the  telescope. 

According  to  Thomassy,f  the  pilot  Ben.  Morgan,  who  has 
visited  it,  describes  it  as  being  "  a  regular  truncated  cone,  20  to 
25  feet  high  and  300  in  circumference,  spouting  at  intervals 
from  its  summit  masses  of  clayey  and  sandy  mud,  which  over- 
flows all  around." 

From  the  Pass,  it  now  appears  as  a  slightly  irregular,  conical 
hill,  which,  judging  from  the  extent  to  which  it  projects  above 
the  highest  reeds,  is  about  18  to  20  feet  high  at  most.  On  its 
eastern  side  there  is  a  second  cone  about  half  as  high,  with  a  very 
regular  slope  not  exceeding  30°,  while  that  of  the  large  one  is 
at  least  45°.  A  whitish  sheen  which  extends  from  the  summit 
of  the  larger  cone  toward  the  smaller  one,  I  interpret  as  a 
white  saltv  efflorescence ;  but  on  the  south  slope  of  the  smaller 

*  CoL  Sidell  mentions  a  mudlump  cone,  18  feet  high,  on  the  north  side  of  the 
Northeast  Pass,  in  the  marsh.  It  has  probably  succumbed  since  his  visit,  as  in 
passing  the  spot  I  was  unable  to  perceive  any  elevation,  nor  was  it  known  to  the 
pilots. 

f  Geol.  prat,  de  la  Louisiane.  p.  56.  T  shall  designate  this  cone  as  'J  Morgan's 
Lump." 


and  the  Mudlumps  of  the  Mississippi.  359 

cone,  the  glittering  of  a  flowing  mudstream  was  unmistakable. 
Inasmuch  as  in  Morgan's  account  of  his  visit  the  smaller  cone 
is  not  mentioned,  it  is  presumable  that  it  has  been  formed  since, 
by  a  lateral  eruption  ;  the  old  cone  having,  perhaps,  reached 
the  extreme  limit  of  height  to  which  mudlump  force  can  raise 
its  materials. 

The  steep  slopes  of  both  the  old  and  new  cone  are  suggestive 
as  to  the  influence  of  sandiness  on  that  feature,  and  the  explana- 
tion of  the  steep  inclination  of  strata,  observed  chiefly  on 
Southwest  Pass. 

It  is  my  impression  that  another  cone  exists  in  the  marsh 
about  two  miles  south  of  these.  It  is  almost  screened  from 
view  by  the  reeds,  but  the  telescope  shows  it  to  be  distinctly 
conical.  It  does  not  seem  to  have  attracted  notice  heretofore. 

But  if  mudlump  cones  are  scarce  in  the  marshes,  the  same 
is  not  true  with  reference  to  the  salt  and  gas  springs,  which  are 
reported  to  be  quite  abundant  by  the  hunters — the  only  men 
whose  occupation  leads  them  to  "thread  the  pathless  waste"  of 
reedy  marsh,  otherwise  seldom  visited,  save  by  surveying  par- 
ties, and  alligators.  These  springs  are  found  on  or  around  all 
mudlumps,  of  whatever  age ;  even  on  the  shoals  left  behind  by 
disintegrated  lumps,  where  they  issue  under  water,  sometimes 
altering  perceptibly  the  character  of  the  water  in  the  immediate 
neighborhood.  The  Southwest  lighthouse  was  originally  built 
on  a  mudlump  separated  from  the  mainland  by  a  bayou  ;  this 
is  now  filled  up,  but  salt  springs  still  issue  at  several  points  in 
the  marsh  near  the  foot  of  the  tower.  It  is  obvious  that  the 
gradual  accumulation  of  deposit  is  not  likely  to  check  lively 
springs,  possessing  sufficient  head  to  rise,  hydrostatically,  above 
the  level  of  the  alluvium  ;  though  in  many  cases  they  may  lose 
themselves  in  the  sandy  strata. 

I  have  not  had  an  opportunity  of  ascertaining  whether  or  not 
salt  springs  are  known  to  exist  in  the  marshes  near  the  Forts, 
and  above.  I  fully  expect  to  find,  however,  that  they  do  exist, 
though,  for  obvious  reasons,  they  will  become  less  and  less 
abundant  as  we  ascend  the  river.  At  New  Orleans,  as  already 
stated,  gas  and  salt  water  are  reached,  and  brought  to  the  sur- 
face with  considerable  vehemence,  by  bores  varying  from  31  to 
56  feet  ;*  and  I  have  no  difficulty  in  believing  in  the  correct- 
ness of  the  impression  made  upon  Col.  Sidell,  that  the  founda- 
tions of  the  New  Orleans  customhouse  were  located  upon  a 
mudlump. f  That  such  obstinate  resistance  as  that  of  the  "  Head 
of  the  Passes  "  to  denudation  can  hardly  be  attributed  to  a  mass 
of  river  deposit,  I  have  already  intimated.  A  large  mudlump 
mass  has,  probably,  first  caused  the  deflection. 

[To  be  continued.] 
*  See  above,  p.  245.  f  Lyell's  Principles  of  Geol.,  10th  ed.,  p.  552 


E.   W.  Hilgard — Geology  of  the  Delta,  etc.  360 


Origin  of  the  Mudlumps. — The  causes  which  give  rise  to  the 
formation  of  mudlumps  have  been  to  some  extent  discussed  by 
Sidell,  Thomassy,  and  Lyell  (loc.  cit.).  The  former  is  inclined  to 
ascribe  the  upheaval  chiefly  to  the  pressure  of  gas  formed  in  the 
decay  of  driftwood  and  the  like,  buried  in  the  river  deposits. 
Thomassy  resorts  to  the  hypothesis  of  the  existence  of  subterra- 
neous channels  communicating  with  the  river,  or  with  equally 
hypothetical  reservoirs  of  water,  far  above  ;  while  I/^ell  ascribes 
the  bulging  of  the  bottom  to  the  pressure  of  newly  formed  de- 
posits upon  a  substratum  of  yielding  mud,  accompanied,  and 
aided  incidentally  only,  by  the  evolution  of  marsh  gas  in  the 
decay  of  organic  matter.  I  myself,  having  become  aware  of 
the  existence  of  a  strong  artesian  water  pressure  in  the  littoral 
formations  of  the  Gulf,  was  inclined  to  ascribe  the  origin  of  the 
upheaving  force  to  that  source  ;  and  my  visit  to  the  mouths  had 
for  its  object  mainly,  the  comparison  of  the  facts  with  each  of 
the  three  admissible  hypotheses,  that  of  Thomassy  being  too 
fanciful  to  be  seriously  entertained. 

As  already  stated,  I  at  once  found  that  the  evolution  of  gas 
in  the  active  vents  was  too  insignificant  to  be  considered  as  the 
cause  of  the  rising  of  the  liquid  mud,  which  so  greatly  exceeded 
it  in  bulk,  that  the  ascensional  force  of  the  bubbles,  especially 
in  so  wide  a  vent-tube,  would  be  utterly  inadequate  to  balance 
the  downward  tendency  of  so  heavy  a  liquid.  It  might  still  be 
alleged,  in  favor  of  the  gas-hypothesis,  that  its  pressure  might 
be  exerted  statically  upon  the  surface  of  the  mass  of  liquid 
mud  covered  by  impervious  strata ;  but  it  is  obvious  that  in 
such  a  case,  the  gas  itself,  necessarily  accumulating  at  the 
highest,  and  therefore  weakest,  points,  of  the  superincumbent 
mass,  would  be  much  more  likely  to  break  through  by  itself, 
promptly  exhausting  its  force  and  quantity  at  any  one  point 
No  such  rushing  outbreaks  of  gas  have  ever  been  recorded, 
save  in  the  case  of  blowing  up  of  a  lump  with  gunpowder;  and, 
as  Lyell  remarks,  this  view  renders  inexplicable  the  occurrence 
of  lumps  exclusively  about  the  mouths  of  the  passes. 

The  latter  objection  applies  equally  to  the  hypothesis  of  the 
artesian  origin  of  mudlump  force,  unless  upon  the  (unproved) 
supposition  that  the  excavation  of  the  river  channel  might 
have  rendered  the  outbreak  of  the  artesian  water  easier  there 
than  elsewhere.  But  instead  of  excavating,  the  Mississippi  has 
for  a  long  time  past  always  thrown  shallows  in  advance  of  its 

AM.  JOUR.  Sci.— THIRD  SERIES,  VOL.  I,  No.  6.— JUNE,  1871. 
28 


361  E.  W.  Hilgard— Geology  of  the  Delta, 

mouths ;  and  unless  it  were  conclusively  proven  that  the  mat- 
ters ejected  by  the  mud-springs  were  such  as  could  not  originate 
in  the  present  delta  formation,  the  artesian  hypothesis  must  lose 
all  show  of  probability.  An  accurate  investigation  of  the  mat- 
ters in  question,  solid,  liquid,  and  gaseous,  was  therefore 
indicated.  A  few  specimens  for  this  purpose  were  collected  for 
me  by  Mr.  Marindin,  in  1867 ;  but  the  perusal  of  his  report 
accompanying  them  convinced  me  that  a  personal  examination 
iTi  loco  could  alone  insure  a  perfect  certainty  as  to  their  signifi- 
cance, and  accordingly,  in  the  autumn  of  the  same  year,  I 
re-collected  specimens  from  the  same,  as  well  as  from  other 
localities.  Yery  unfortunately,  the  arrangements  for  gas  analy- 
sis at  my  command  were  so  imperfect  that,  while  waiting  for 
their  improvement,  the  gas  specimens  were  so  vitiated  by  diffu- 
sion through  corks  and  wax  as  to  render  them  useless ;  and  I 
have  been  unable  to  replace  them  as  yet,  but  hope  to  do  so  in 
the  near  future. 

Mudlump  Gases. — The  examination  of  the  water  and  mud 
seemed,  however,  most  likely  to  conduce  to  a  solution  oi  the 
problem,  at  any  rate ;  for  after  all,  the  only  information  which 
could  be  furnished  by  gas  analysis  would  be  to  indicate,  by  the 
greater  or  less  amount  of  carbonic  acid  present,  whether  the  gas 
originated  from  matter  comparatively  fresh  and  in  its  first  stage 
of  decomposition,  or  had  its  source  in  materials  far  advanced 
toward  the  stage  of  lignite  or  coals.  The  only  perfectly  reliable 
determination  made  was  that  of  the  carbonic  acid  contained  in 
the  gas  collected  from  the  most  easterly  active  cone  on  Marin- 
din'sLump,  Passeal'Outre,  the  rest  of  the  determinations  being 
somewhat  vitiated,  though  doubtless  very  nearly  correct*  The 
result  was  as  follows  : 

Gas  from  East  Crater,  Marindin's  Lump,  Passe  d  V  Outre. 

Carbonic  acid, 9'41 

Marsh  gan, 86'20 

Nitrogen, 4'39 

]  00-00 

Oxygen  was  not  present. 

The  percentage  of  carbonic  acid  in  this  gas. is  very  unusually 
large ;  its  composition  is  nearest  to  that  of  the  gas  from  com- 
mon swamps,  where  vegetable  matter  is  in  its  first  stages  of  de- 
cay. The  proportion  between  marsh  gas  and  nitrogen  is  nearly 
the  same  as  in  the  gas  from  the  gas  wells  at  New  Orleans  (see 
p.  245) ;  but  there  is  three  times  as  much  carbonic  acid  present 

*  After  the  explosion  in  the  eudiometer,  some  nitrate  of  mercury  was  obserred  on 
its  walls,  in  consequence  of  inadequate  dilution  of  the  gas.  But  the  marsh  gas 
was  estimated  from  the  carbonic  acid  absorbed  after  the  explosion,  the  nitrogen  by 
difference. 


and  the  Mudlumps  of  the  Mississippi.  362 

in  the  mud-lump  gas,  in  accordance  with  the  presumable  more 
advanced  stage  of  decay  existing  in  the  former  locality. 

Mudlump  Spring  Waters. — In  taking  specimens,  common 
quart  bottles  were  filled  by  immersion  in  the  craters  them- 
selves, and  irfcmediately  sealed.  The  liquid  mud  thus  obtained 
would,  after  a  while,  separate  into  a  lower  stratum  of  pretty 
solid  mud,  and  an  upper  one  of  clear  water,  in  varying  propor- 
tions. For  analysis,  the  latter  was  carefully  decanted,  and  the 
turbid  part  rapidly  filtered  through  a  Bunsen's  pressure  filter, 
and  measured.  400  ccm.  were  then  boiled  to  precipitate  car- 
bonates and  silica,  the  filtrate  re-diluted  to  the  original  bulk, 
and  from  50  to  100  ccm.  used  in  the  determination,  in  separate 
portions,  of  chlorine,  of  lime  and  magnesia,  and  of  sulphuric 
acid  and  alkalies,  respectively ;  while  a  fourth  portion  served 
for  an  approximate  determination  of  the  solid  residue,  for  the 
sake  of  roughly  controlling  the  final  results.* 

I  give  below,  in  tabular  form,  the  results  of  these  analyses  ; 
presented  in  three  different  forms,  for  the  sake  of  ready  com- 
parison with  the  composition  of  sea-water,  from,  which  they 
seem  to  be  derived  by  a  series  of  reactions  easily  understood 
from  the  nature  and  condition  of  the  materials  with  which  they 
are  associated. 

I.  Water  from  the  basin  of  a  spring  on  a  mudlump  off  Stake 
Island,  Southwest  Pass.     Evolves  gas  and  water  in  about  equal 
proportions,  no  mud,   but  only  fine  sandy  matter ;  and  water- 
flows  off  clear  over  the  rim  of  the  basin,  wnich  is  two  feet  above 
tide  level,  and  at  the  foot  of  a  large  extinct  cone  with  a  lagoon, 
surrounded  by  a  high  rim,  in  the  center. 

Water  about  J  of  the  bulk  in  bottle,  the  rest  fine  sand. 
Taste,  very  salty  ;  color,  slightly  yellowish ;  turns  brownish 
turbid  very  quickly  on  exposure  to  air.  Coll.  Dec.  3,  1867. 

II.  Water  from  a  mudlump  spring  on  Northeast  Pass,  col- 
lected by  H.  L.  Marindin,  of  U.  S.  Coast  Survey  schr.  Varina, 
in  February,  1867. 

According  to  the  recollection  of  one  of  the  crew,  this  speci- 
men was  taken  from  the  same  cone  as  the  following  one  (No. 

in). 

Water  clear,  faintly  yellowish,  about  J  by  bulk  of  the  con- 
tents ;  the  rest  sandy  mud.  Turns  turbid  rapidly  on  exposure 
to  air. 

*  With  mixtures  of  this  kind  no  method  but  that  of  evaporation  with  excess  of 
carbonate  of  soda  will  yield  anything  more  than  an  approximate  estimate  of  the 
solid  residue;  involving  an  amount  of  labor  and  care  uot  always  justified  by  the 
end  in  view,  when  the  relative  amounts  of  ingredients  can  serve  to  control.  The 
chlorine  determination  being  the  most  accurate,  and  almost  always  in  excess  of  the 
bases  found  available  to  form  chlorides,  the  chloride  of  sodium,  as  here  recorded, 
is  the  calculated  amount,  as  is  also  the  sum  of  ingredients. 


363 


K   W.  Hilgard— Geology  of  the  Delta, 


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and  the  Mudlumps  of  the  Mississippi. 


364 


TABLE— continued. 


Knight's  Gas  Well. 
New  Orleans. 

Gulf  Water. 
30  miles  out. 

Sea  Water. 
Average  com- 
position. 

VI. 

VII. 

vm. 

a. 

b. 

c. 

a. 

b. 

a. 

b. 

Chloride  of  sodium,  

0-02310 

37-880 

56-479 

1  6723 

76-870 

2-700 

77-032 

potassium,  _ 
"           calcium,  

0-00790 
0-00060 

12-950 
0-980 

19-315 
1467 

0-0357 

1-640 

0-070 

1-997 

"           magnesium,  - 
Sulphate  of  calcium,  .  . 

0-00630 

10-330 

15-404 

0-2310 

10-620 

0-360 

10-271 

Bromide  of  magnesium, 
Sulphate 

0-00300 

4-920 

7-335 

trace. 
0-1077 
0-1289 

trace. 
4-950 
5-920 

0-002 
0-140 
0-230 

0-058 
3-994 
6-562 

Carbonate  of  calcium,  _  0-01120 
'•          magnesium,  |0'00760 
"          iron  

18-360 
12450 

2-130 

0-003 

0-086 

Silica, 

0-00130 

0-06LOO 

100-000 

100-000 

2-1756 

100-000 

3-505 

100-000 

Specific  gravity,  .  _ 

1-01630 

1-0298 

III.  Water  from  faintly  active  cone  on  Salt  Spring  Island,  off 
Northeast  Pass ;  from  crater  on  west  side  of  island,  about  ten 
feet  above  sea-level.     Collected  Dec.  2,  1867. 

Water  very  salty,  about  f  of  the  whole  mass;  the  rest,  a 
somewhat  sandy  mud ;  color,  slightly  yellowish ;  turns  turbid 
rapidly  on  exposure  to  air. 

The  larger  proportion  of  water  in  this  specimen,  as  compared 
with  the  preceding,  doubtless  results  from  the  comparative  in- 
activity of  the  cone,  as  compared  to  that  at  the  time  of  high 
water  in  February.  The  same  fact  may  account  for  some  of 
the  difference  in  composition. 

IV.  Water  from  East  Crater  on  Marindin's  Lump,  Passe  a 
1'Outre.     See  p.  362.     Collected  Dec.  2,  1867. 

Forms  about  f  of  the  bulk  in  the  bottle,  the  rest  is  soft 
clayey  mud.  Faintly  brownish,  clear  ;  becomes  brownish  tur- 
bid rapidly  on  exposure  to  air. 

V.  Water  from  West  Crater  on  Marindin's  Lump,  Passe  a 
1' Outre ;  same  date. 

Water  about  J  of  bulk  in  bottle,  the  rest  clayey  mud.  Not 
very  salty  ;  colorless,  but  turns  turbid  quickly  on  exposure  to 
air. 

VI.  Water  from  gas  well  bored  by  J.  B.  Knight,  at  New  Or- 
leans.    Clear,  with  a  little  sand  at  bottom ;  taste,  faintly  brack- 
ish. 

VII.  Water  of  the  Gulf  of  Mexico,  taken  from  surface  30 
miles  out,  southeast  from  Southwest  Pass,  Dec.  1867. 

VIII.  Average  composition  of  sea-water,  according  to  Keg- 
nault. 

The  general  results  deducible  from  the  above  analyses  may 
be  thus  stated : 


365  E.  W.  Hilyard—  Geology  of  the  Delta, 

1.  There  is  a  general  similarity  of  composition  between  the 
waters  of  the  mudlump  springs,  indicating  their  derivation  from 
a  common  source  of  supply.     But  springs  on  the  same  lump,  as 
well  as  the  same  spring  at  different  times  or  stages  of  water,  may 
vary  quite  sensibly,  both  in    composition  and  concentration. 
Their  density  is  generally  inferior  to  that  of  sea-water,  though 
at  times  approaching  it  closely. 

2.  There  is  an  obvious  approximation  of  the  ratio  between 
the  two  chief  bases — sodium  and  magnesium — to  that  existing 
in  sea- water ;  the  variations  being  no  greater  than  are  observed 
in  sea-water  from  different  localities. 

3.  The  most  obvious  difference  is  the  absence  of  sulphates, 
and  their  partial  replacement  by  chlorides  ;  also 

4.  The  presence  of  large  amounts  of  the  carbonates  of  the 
earths,  as  well  as  of  iron,  dissolved  in  carbonic  acid. 

5.  The  amount  of  potassium  salts  is  decidedly  diminished. 

6.  Bromids  appear  to  be  entirely  absent,  and  are  present  in 
traces  only,  in  the  water  of  the  Gulf  itself 

It  is  stated  that  the  waters  of  some  of  the  springs  are  fresh. 
I  have  found  all  brackish  at  least,  and  as  the  population  of  the 
delta  is  much  in  the  habit  of  drinking  water  of  questionable 
freshness,  their  judgment  in  the  matter  may  not  be  the  most  re- 
liable. 

As  regards  the  water  of  the  New  Orleans  well,  though  at  first 
sight  it  differs  materially  from  the  others,  it  will  be  observed 
that  when  in  C,  potassium  and  sodium  chlorides  arc  classed 
together,  its  composition  becomes  not  unlike  the  mudlump  wa- 
ters. Its  great  dilution  accounts  for  the  excessive  proportion  of 
carbonates. 

The  Grulf  water  approaches  very  closely  in  its  composition 
to  the  normal  one  of  sea-water,  as  given  by  Eegnault 

Mud  from  Mudlump  Springs. — I  have  already  stated,  that  the 
stratified  material  of  the  mudlumps  (whether  formed  by  the  ac- 
tion of  mud  springs,  or  bodily  upheaved)  is  free  from  visible 
shells  or  other  fossils,  save  particles  of  woody  matter ;  while 
the  amorphous  material  which  forms  the  surface  and  generally 
also  the  beach,  often  abounds  in  such  organic  remains  as  are 
now  usually  washed  ashore,  having  manifestly  been  cast  up  and 
imbedded  in  the  mud  by  the  waves. 

I  have  examined  microscopically  the  mud  remaining  in  the 
specimen  bottles  after  decanting  the  water  for  analysis;  they 
were  immediately  refilled  with  distilled  water,  and  kept 
closely  stopped  until  examined.  I  give  below  the  record  of 
examination  of  the  mud,  corresponding  to  analysis  No.  Y, 
from  West  Crater  on  Marindin's  Lump.  Two  or  three  others 
similarly  examined  gave  a  like  result. 


and  the  Mudlumps  of  the  Mississippi.  366 

A  sample  from  the  general  mass  shows  it  to  consist  mainly 
of  very  fine  quartz  sand,  mostly  angular,  with  but  a  few  large, 
angular  grains ;  and  but  little  true  clay. 

In  washing  the  mass,  even  the  first  washings  were  found  to 
contain  but  little  clay,  but  chiefly  very  fine  suspended  silex. 
A  few  ill  defined  spicules,  a  Navicula,  and  bark  fibers. 

In  the  middle  portion,  spicules  a  little  more  abundant. 

In  the  coarsest  portion,  much  variously  colored  mica,  along 
with,  mostly  sharply  angular,  quartz  grains  ;  numerous  particles 
of  water-browned  wood  ;  very  distinct  spine  of  a  radiate  ;  sev- 
eral specimens  of  Rotalina,  and  fragments  of  same  as  well  as 
Uvigerina,  Cristellaria,  Amphistegina,  and  Coccineis  ?  Also 
iridescent  fragments,  showing  lines  of  growth,  from  the  edges 
of  larger  bivalves. 

Quartz  grains  mostly  transparent  and  angular ;  some  of 
milky,  oil-green,  and  rose  quartz  ;  these  mostly  rounded.  This 
sand  resembles  closely  that  of  the  bottom  outside  Northeast 
Pass  bar,  in  40  to  50  feet  water ;  specimens  of  which  were  fur- 
nished me  by  the  Coast  Survey  party,  under  command  of  Capt. 
F.  V.  Webber  of  the  schooner  Varina,  in  1869. 

Specimens  from  mudlumps  on  Southwest  Pass  show  coarser 
sand,  and  rather  more  Foraminifera. 

The  character  of  the  materials  ejected  by  the  mudlump 
springs,  as  determined  by  the  foregoing  investigations,  may  be 
summed  up  as  follows  : 

1.  The  gas  is  such  as  is  evolved  by  vegetable  matter  in  its 
first  stages  of  decay  or  lignitization. 

2.  The  earthy  matter  contains  both  river  and  marine  fossils — 
driftwood  reduced  almost  to  its  cell- elements  by  maceration  and 
trituration,  as  well  as  Foraminifera.     Its  fineness  is  such  that, 
before  final  deposition,  it  may  have  been  carried  out  into  water 
of  considerable  depth. 

3.  The  mudlump  waters  appear  to  be  sea-water  more  or  less 
diluted,  and  chemically  changed  under  the  joint  influence  of 
fermenting  organic  matter,  and  the  more  active  ingredients  of 
the  river  deposit,  viz:  carbonates  of  lime  and  magnesia,  and 
oxide  of  iron. 

The  first  effect  thus  produced  would  probably  be  the  addition 
of  the  soluble  carbonates  of  these  metals  to  the  solution.  But 
the  soluble  sulphates  could  not,  in  the  presence  of  a  soluble 
iron  salt,  long  resist  the  reducing  influence  of  decaying  organic 
matter.  As  usual  under  such  circumstances,*  iron  pyrites 
would  be  formed,  withdrawing  in  the  end  all  the  sulphuric  acid 
from  the  solution,  and  forming,  instead,  equivalent  amounts  of 
the  respective  carbonates.  The  amounts  of  the  chlorides  of 

*  See  Bischoff,  Chemische  G-eologie,  vol.  i,  p.  559. 


367  E.   W.  Hilgard— Geology  of  the  Delta, 

sodium  and  magnesium  originally  present  would  thus,  also,  be 
relatively  increased  ;  and  this  again  tallies  with  the  analytical 
results.  Yet  while  the  proportion  between  these  bases  is  main- 
tained* the  actual  replacement  of  sulphate  of  magnesium  by  the 
chloride  constitutes  a  change  not  readilv  explained.  At  first 
sight  it  would  seem  that  the  excess  of  chlorine  belonging  to  the 
chlorides  of  calcium  and  magnesium  must  be  an  outside  acces- 
sion ;  but  our  knowledge  of  the  mutual  reactions  between  the 
substances  here  present  under  strong  pressure,  is  perhaps  too 
fragmentary  to  justify  an  assertion  on  this  point. 

The  diminution  of  the  potash  salts  is  doubtless  referable  to 
their  absorption  by  the  clays  present,  in  preference  to  all  other 
compounds.  The  filtration  of  sea- water  through  soil  would,  ac- 
cording to  Liebig's  experiments,  produce  a  like  result. 

Conclusions. — In  view  of  the  foregoing  facts,  the  explanation 
of  mudlump  phenomena  suggested,  substantially,  by  Sir  Chas. 
Lyell,  seems  the  only  tenable  one ;  it  requires,  however,  some 
modification  as  regards  the  mass  supposed  to  exert  the  pressure, 
and  some  corollaries  as  to  the  mode  of  action. 

I  have  before  suggested,  that  the  rapid  protrusion  of  the 
mouths  of  the  river  into  the  Gulf,  in  advance  of  the  body  of 
the  delta,  is  owing  to  the  shelf  of  "  blue  clay  bottom  "  extending, 
at  a  comparatively  slight  depth  below  the  sea-level,  and  with  a 
gentle  slope,  far  out  into  the  Gulf.  It  is  upon  this  impervious 
clay  that  the  present  bars  are  based  ;  and  upon  it,  in  advance 
of  the  bar,  will  be  deposited  the  finest  of  the  river  mud,  at  a 
depth  at  which,  perhaps,  the  sea-water  is  at  the  time  quite 
undiluted,  and  fully  adapted  to  marine  life  ;  which  will  there- 
fore deposit  its  vestiges  in  it,  associated  with  the  finest  particles 
of  driftwood,  etc.  Mud  thus  deposited  may  remain  unconsoli- 
dated  for  a  great  length  of  time,  unless  the  water  be  drained 
off  or  through  it  by  some  means. 

The  bar,  in  its  annual  advance  of  about  338  feet,  will  cover 
over  this  liquid  mud  stratum,  exerting  a  pressure  much  greater 
than  that  of  the  sea  ;  and  were  it  resting  on  a  pervious  bottom, 
the  liquid  mud  would  doubtless  soon  be  consolidated  into  a 
sheet  of  clay.  As  it  is,  the  tendency  will  be  to  squeeze  it 
from  under  the  crest  of  the  bar,  both  seaward  and  landward. 
The  very  gradual  seaward  slope  of  the  bar  will  render  the 
movement  in  that  direction  a  very  slow  one,  under  a  greater 
depth  of  water  and  heavy  frictional  resistence.  Not  so  on  the 
land  side,  where,  as  the  bar  advances,  the  superincumbent  pres- 
sure is  measurably  relieved  by  the  erosion  of  a  channel  by  the 


*  The  ratio  is  for 

Atlantic  sea-water,  30 '5  :  3'0 

German  Ocean  water,  30-5  :  4'0 

Average  sea         "  30'5  :  3*4 


Marindin's  Lump  water,       30-5  :  2'8     (min.) 
Southwest  Mudlump,  30'5  :  3*56  (max.) 

Average  of  those  analyzed,  30*5  :  31 


and  the  Mudlumps  of  the  Mississippi.  368 

current,  which,  especially  in  flood  time,  carries  much  of  the 
bottom  deposit  bodily  over  the  bar  and  drops  it  to  seaward.* 
At  weak  points  of  the  bottom  inside  the  bar,  therefore,  the 
upward  pressure  of  the  mud  may  cause  a  bulging  up  at  least 
to  the  level  of  the  bar-crest,  and,  perhaps,  taking  into  account 
the  difference  in  the  specific  gravities  of  the  comparatively 
solid  bar  and  liquid  mud,  even  as  far  as  the  surface  of  the 
water.  But  this,  considering  the  question  as  coming  within 
the  domain  of  liquid  statics,  would  seem  to  be  about  the  ex- 
treme limit  to  which  the  bottom  itself  could  be  brought  up. 

I  have  found  the  specific  gravity  of  the  mud  .flowing  from 
the  West  Crater  on  Marindin's  Lump,  to  be  about  1 '25 ;  that  from 
the  cone  on  Salt  Spring  Island,  on  Northeast  Pass,  1'30  ;  while 
that  of  bar  deposit  from  the  crest  of  the  Northeast  bar,  wet  as 
brought  up  by  the  lead,  was  175.  The  heights  of  communi- 
cating columns  of  these  substances,  if  sensibly  liquid,  should 
be  as  5  to  7  ;  but  this  ratio  could  apply,  in  the  case  of  upheaval, 
only  so  far  as  the  difference  of  level  between  the  bar  and  the 
upheaved  bottom  is  concerned,  since  the  latter  must  be  pre- 
sumed to  be  similar  to  the  bar  in  its  materials  and  structure. 
Should  the  current,  however,  continue  to  denude  the  crest  of 
the  upheaved  mass,  the  rising  would  continue  and  the  semi-fluid 
mud  might  finally  break  through,  forming  a  mudspring,  the 
height  of  whose  vent  above  its  source  might  finally  increase  to 
the  extent  corresponding  to  the  difference  of  specific  gravity. 

The  craters  of  mudlumps  have  been  sounded  to  the  depth  of 
24  feet,  but  no  precautions  were  used  to  insure  reaching  the 
actual  maximum  depth.  In  the  borings  made  for  the  founda- 
tion of  a  lighthouse  on  the  Southwest  Pass,  by  Mr.  A.  Palms 
(the  record  of  which  was  courteously  forwarded  to  me  by  the 
Engineer  Dept),  a  stratum  so  soft  that  the  augur  sank  in  'it  by 
its  own  weight,  was  met  with  at  58  ft.,  after  striking,  at  56  ft. 
a  stream  of  water  which  "filled  the  pipe." 

If  then,  the  mud  stratum,  lying,  say  60  ft.,  below  the  surface, 
be  pressed  by  a  column  of  deposit  of  1*75  sp.  gr.,  mud  of 
sp.  gr.  1-25  could  thereby  be  raised  24  feet  above  the  top  of 
the  pressing  column  ;  and  this,  considering  the  average  depth 
of  water  on  the  bar,  would  account  for  the  greatest  heights  to 
which  cones  are  built  up  off  the  mouths. 

But  this  is  a  close  calculation,  even  if  the  data  upon  which 
it  is  based  be  deemed  admissible  in  the  form  I  have  given  them ; 
and  the  frequency  and  energy  with  which  the  upheaving  force 
acts,  coupled  with  the  fact  that  when  a  mudlump  rises  in  the 
channel,  so  far  from  suffering  denudation  to  the  extent  re- 
quired for  the  breaking  through  of  the  mud,  it  more  generally 
causes  a  silting  up  of  the  channel :  seems  to  me  to  indicate  that 

*  Humphreys  and  Abbot's  Kept.,  p.  446. 


369  E.   W.  Hilgard— Geology  of  the  Delta,  etc. 

a  stronger  force,  less  delicately  balanced  than  the  equilibrium 
of  the  bar,  is  "  at  the  bottom  "  of  the  whole  phenomenon. 

This  force,  I  think,  is  to  be  sought  in  the  constantly  increas- 
ing weight  of  the  alluvial  area  above  the  mouths,  which,  itself 
possessing  a  series  of  mudlump  vents  at  one  time,  must  yet  be 
resting  in  a  great  measure  upon  the  still  unexhausted  mud 
stratum ;  as  is  proven  by  the  existence  of  active  lumps  in  the 
marshes,  even  though  the  increased  resistance  of  superincum- 
bent deposit  as  well  as  matted  vegetation  must  render  their 
occurrence  there  a  rarity.  There  must  still  be  a  communica- 
tion of  liquid  pressure  "between  the  older  and  newer  portions 
of  the  modern  delta ;  and  this  point  is  especially  strengthened 
by  the  fact  that  a  high  stage  of  water  in  the  river,  which  does 
not  sensibly  affect  the  depth  on  the  bars,  yet  exerts  a  decided 
influence  on  mudlump  activity.  The  river  not  only  overflows 
the  marshes,  but  loads  them  with  additional  sediment;  and 
doubtless  the  increased  hydrostatic  pressure  stops  many  a  vent 
of  gas,  mud  or  water,  which  ordinarily  discharges  into  the 
river's  bed.* 

Morgan's  Lump,  in  the  marsh  of  Southwest  Pass,  and 
Marindin's  Lump  on  Passe  a  1'Outre,  are  now  known  to  have 
been  in  undiminished  activity  for  twenty-five  years  at  least. 
Since  that  time  the  bars  have  moved  gulfward  a  mile  and  a 
half;  and  one  would  think  that,  if  the  activity  of  the  cones 
depended  upon  them  alone,  a  notable  difference  ought  to  have 
been  observed.  But  if  the  main  force  is  a  vis  a  tergo,  while 
the  bar  serves  mainly  to  prevent  the  escape  of  the  mud  to 
seaward,  there  is  good  cause  for  the  secular  persistence  of  vents 
that  have  escaped  mechanical  obstruction. 

How  far  above  the  present  mouths  the  head  of  pressure  may 
extend,  I  do  not  pretend  to  conjecture.  The  borings  at  New 
Orleans  seem  to  indicate  that  the  mud  stratum  originally 
existed  there  also,  but  it  would  be  extravagant  to  suppose  that 
such  pressure  as  that  exerted  in  the  gas  wells  of  that  city,  could 
now  be  felt  a  hundred  miles  below.  Yet  it  seems  not  at  all 
unlikely,  that  the  weight  which  steadily  forces  up  the  liquid 
mud  to  the  top  of  Morgan's  Lump,  seven  miles  above  the 
mouth,  may,  in  part,  be  furnished  by  the  enormous  mass  of 
vegetation  which  annually  develops  in  the  marshes,  willow 
battures,  and  perhaps  even  cypress  swamps  above.  Nor  is  the 
effect  of  gaseous  pressure  resulting  from  the  constantly  pro 
gressing  decay  of  organic  matter  to  be  overlooked,  although  I 
doubt  that  this  cause  plays,  ordinarily,  anything  more  than  a 
very  subordinate  part. 

*  I  give  on  the  plate  (page  358)  an  ideal  section,  illustrating  this  explanation  of 
the  "origin  of  mudlumps." 


and  the  Mudlumps  of  the  Mississippi.  435 

Future  observations,  systematically  carried  out,  will  doubtless 
solve  a  good  many  of  the  questions  here  mooted ;  and  though 
they  may  not  lead  to  the  suggestion  of  any  means  whereby 
the  k'  evil  geniuses  of  the  Passes  "  may  at  present  be  conjured, 
a  more  precise  knowledge  of  data,  as  well  as  of  the  statics  and 
dynamics  of  mud,  may  enable  us  to  predict  at  what  point  of 
advance  of  the  mouths  into  the  deeper  water  of  the  Gulf,  their 
formation  must  cease.  The  Southwest  Pass  appears  to  be 
nearest  that  consummation  devoutly  to  be  .wished  ;  and  were 
the  closing  of  the  other  outlets  practicable,  the  advance  of  the 
Southwest  bar  might  become  so  rapid,  as  to  let  the  youngest 
of  the  living  generation  witness  a  diminution  of  mudlurnp 
upheaval.  Ultimately,  the  mouth  might  thus  become  similar 
to  those  of  the  Orinoco  and  Amazon  ;  but  until  then,  ceaseless 
activity  of  the  river  in  the  formation  of  bars  and  mudlumps 
must,  in  the  interest  of  navigation  and  commerce,  be  met 
by  an  equally  ceaseless  and  diligent  effort  for  their  removal 
from  the  channel.  For  while  a  concentration  of  the  river  cur- 
rent might  possibly  be  made  to  maintain  the  needful  depth  upon 
the  bars,  its  utmost  erosive  energy  will  be  powerless  against  the 
tough,  inert  masses  of  the  mudlumps. 


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